LibGUI: Add GModelSelection to help implementing multiple-select views

Each GAbstractView now has a GModelSelection backed by a simple
HashTable<GModelIndex>. When the selection changes somehow, the view
gets notified via the notify_selection_changed() callback.

In the future it will probably make sense to move to using some kind of
ranges as the internal representation instead.

Libraries/LibGUI/GAbstractView.cpp

@@ -8,6 +8,7 @@
 
 GAbstractView::GAbstractView(GWidget* parent)
     : GScrollableWidget(parent)
+    , m_selection(*this)
 {
 }
 
@@ -96,3 +97,8 @@ void GAbstractView::activate(const GModelIndex& index)
     if (on_activation)
         on_activation(index);
 }
+
+void GAbstractView::notify_selection_changed(Badge<GModelSelection>)
+{
+    update();
+}