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new file mode 100644
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Binary files /dev/null and b/data/img/material/black/minus.png differ
diff --git a/data/img/material/black/minus.svg b/data/img/material/black/minus.svg
new file mode 100644
index 00000000..65418b43
--- /dev/null
+++ b/data/img/material/black/minus.svg
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+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   inkscape:version="1.0 (4035a4fb49, 2020-05-01)"
+   sodipodi:docname="minus.svg"
+   id="svg4"
+   version="1.1"
+   viewBox="0 0 24 24">
+  <metadata
+     id="metadata10">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title></dc:title>
+      </cc:Work>
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+  <defs
+     id="defs8" />
+  <sodipodi:namedview
+     inkscape:current-layer="svg4"
+     inkscape:window-maximized="1"
+     inkscape:window-y="0"
+     inkscape:window-x="1920"
+     inkscape:cy="13.075889"
+     inkscape:cx="14.554259"
+     inkscape:zoom="22.388564"
+     inkscape:pagecheckerboard="false"
+     showgrid="false"
+     id="namedview6"
+     inkscape:window-height="1027"
+     inkscape:window-width="1920"
+     inkscape:pageshadow="2"
+     inkscape:pageopacity="0"
+     guidetolerance="10"
+     gridtolerance="10"
+     objecttolerance="10"
+     borderopacity="1"
+     bordercolor="#666666"
+     pagecolor="#ffffff"
+     inkscape:document-rotation="0" />
+  <path
+     style="stroke-width:1.65234;fill:#000000"
+     sodipodi:nodetypes="ccccc"
+     id="path2"
+     fill="#ffffff"
+     d="m 21.914037,10.34766 v 3.30468 c -7.15882,0 -15.2248669,0 -19.8280741,0 v -3.30468 c 6.609358,0 13.2187161,0 19.8280741,0 z" />
+</svg>
diff --git a/data/img/material/black/plus.png b/data/img/material/black/plus.png
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diff --git a/data/img/material/black/plus.svg b/data/img/material/black/plus.svg
new file mode 100644
index 00000000..98a97af7
--- /dev/null
+++ b/data/img/material/black/plus.svg
@@ -0,0 +1,57 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   viewBox="0 0 24 24"
+   version="1.1"
+   id="svg4"
+   sodipodi:docname="plus.svg"
+   inkscape:version="1.0 (4035a4fb49, 2020-05-01)">
+  <metadata
+     id="metadata10">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title></dc:title>
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <defs
+     id="defs8" />
+  <sodipodi:namedview
+     inkscape:document-rotation="0"
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1"
+     objecttolerance="10"
+     gridtolerance="10"
+     guidetolerance="10"
+     inkscape:pageopacity="0"
+     inkscape:pageshadow="2"
+     inkscape:window-width="1920"
+     inkscape:window-height="1027"
+     id="namedview6"
+     showgrid="false"
+     inkscape:pagecheckerboard="false"
+     inkscape:zoom="23.066184"
+     inkscape:cx="14.554259"
+     inkscape:cy="13.075889"
+     inkscape:window-x="1920"
+     inkscape:window-y="0"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="svg4" />
+  <path
+     d="M 13.65234,2.0859629 V 10.34766 h 8.261697 c 0,3.30468 0,0 0,3.30468 H 13.65234 v 8.261697 c -3.30468,0 0,0 -3.30468,0 V 13.65234 H 2.0859629 c 0,-3.30468 0,0 0,-3.30468 H 10.34766 V 2.0859629 c 3.30468,0 0,0 3.30468,0 z"
+     fill="#ffffff"
+     id="path2"
+     sodipodi:nodetypes="ccccccccccccc"
+     style="stroke-width:1.65234;fill:#000000" />
+</svg>
diff --git a/data/img/material/white/minus.png b/data/img/material/white/minus.png
new file mode 100644
index 00000000..1ddb09ee
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diff --git a/data/img/material/white/minus.svg b/data/img/material/white/minus.svg
new file mode 100644
index 00000000..e02d3235
--- /dev/null
+++ b/data/img/material/white/minus.svg
@@ -0,0 +1,57 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   viewBox="0 0 24 24"
+   version="1.1"
+   id="svg4"
+   sodipodi:docname="minus.svg"
+   inkscape:version="1.0 (4035a4fb49, 2020-05-01)">
+  <metadata
+     id="metadata10">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title />
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <defs
+     id="defs8" />
+  <sodipodi:namedview
+     inkscape:document-rotation="0"
+     pagecolor="#000000"
+     bordercolor="#666666"
+     borderopacity="1"
+     objecttolerance="10"
+     gridtolerance="10"
+     guidetolerance="10"
+     inkscape:pageopacity="0"
+     inkscape:pageshadow="2"
+     inkscape:window-width="1920"
+     inkscape:window-height="1027"
+     id="namedview6"
+     showgrid="false"
+     inkscape:pagecheckerboard="false"
+     inkscape:zoom="23.066184"
+     inkscape:cx="14.554259"
+     inkscape:cy="13.075889"
+     inkscape:window-x="1920"
+     inkscape:window-y="0"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="svg4" />
+  <path
+     d="m 21.914037,10.34766 v 3.30468 c -7.15882,0 -15.2248669,0 -19.8280741,0 v -3.30468 c 6.609358,0 13.2187161,0 19.8280741,0 z"
+     fill="#ffffff"
+     id="path2"
+     sodipodi:nodetypes="ccccc"
+     style="stroke-width:1.65234" />
+</svg>
diff --git a/data/img/material/white/plus.png b/data/img/material/white/plus.png
new file mode 100644
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diff --git a/data/img/material/white/plus.svg b/data/img/material/white/plus.svg
new file mode 100644
index 00000000..f8b8da45
--- /dev/null
+++ b/data/img/material/white/plus.svg
@@ -0,0 +1,56 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   inkscape:version="1.0 (4035a4fb49, 2020-05-01)"
+   sodipodi:docname="plus.svg"
+   id="svg4"
+   version="1.1"
+   viewBox="0 0 24 24">
+  <metadata
+     id="metadata10">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title></dc:title>
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <defs
+     id="defs8" />
+  <sodipodi:namedview
+     inkscape:current-layer="svg4"
+     inkscape:window-maximized="1"
+     inkscape:window-y="0"
+     inkscape:window-x="1920"
+     inkscape:cy="13.075889"
+     inkscape:cx="14.554259"
+     inkscape:zoom="23.764313"
+     inkscape:pagecheckerboard="false"
+     showgrid="false"
+     id="namedview6"
+     inkscape:window-height="1027"
+     inkscape:window-width="1920"
+     inkscape:pageshadow="2"
+     inkscape:pageopacity="0"
+     guidetolerance="10"
+     gridtolerance="10"
+     objecttolerance="10"
+     borderopacity="1"
+     bordercolor="#666666"
+     pagecolor="#000000" />
+  <path
+     style="stroke-width:1.65234"
+     sodipodi:nodetypes="ccccccccccccc"
+     id="path2"
+     fill="#ffffff"
+     d="M 13.65234,2.0859629 V 10.34766 h 8.261697 c 0,3.30468 0,0 0,3.30468 H 13.65234 v 8.261697 c -3.30468,0 0,0 -3.30468,0 V 13.65234 H 2.0859629 c 0,-3.30468 0,0 0,-3.30468 H 10.34766 V 2.0859629 c 3.30468,0 0,0 3.30468,0 z" />
+</svg>