add sidepanel button to make UI more intuitive and accessible without keyboard (tablets, etc.) (#631)

* add sidepanel toggle button and close button #613

* add side panel toggle button option

* make sidepanel button draggable

src/widgets/draggablewidgetmaker.cpp

@@ -0,0 +1,81 @@
+// Copyright(c) 2017-2019 Alejandro Sirgo Rica & Contributors
+//
+// This file is part of Flameshot.
+//
+//     Flameshot is free software: you can redistribute it and/or modify
+//     it under the terms of the GNU General Public License as published by
+//     the Free Software Foundation, either version 3 of the License, or
+//     (at your option) any later version.
+//
+//     Flameshot is distributed in the hope that it will be useful,
+//     but WITHOUT ANY WARRANTY; without even the implied warranty of
+//     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+//     GNU General Public License for more details.
+//
+//     You should have received a copy of the GNU General Public License
+//     along with Flameshot.  If not, see <http://www.gnu.org/licenses/>.
+
+#include "draggablewidgetmaker.h"
+#include <QMouseEvent>
+
+DraggableWidgetMaker::DraggableWidgetMaker(QObject* parent)
+  : QObject(parent)
+{}
+
+void
+DraggableWidgetMaker::makeDraggable(QWidget* widget)
+{
+  widget->installEventFilter(this);
+}
+
+bool
+DraggableWidgetMaker::eventFilter(QObject* obj, QEvent* event)
+{
+  auto widget = static_cast<QWidget*>(obj);
+
+  // based on https://stackoverflow.com/a/12221360/964478
+  switch (event->type()) {
+    case QEvent::MouseButtonPress: {
+      auto mouseEvent = static_cast<QMouseEvent*>(event);
+
+      m_isPressing = false;
+      m_isDragging = false;
+      if (mouseEvent->button() == Qt::LeftButton) {
+        m_isPressing = true;
+        m_mousePressPos = mouseEvent->globalPos();
+        m_mouseMovePos = m_mousePressPos;
+      }
+    } break;
+    case QEvent::MouseMove: {
+      auto mouseEvent = static_cast<QMouseEvent*>(event);
+
+      if (m_isPressing) {
+        QPoint widgetPos = widget->mapToGlobal(widget->pos());
+        QPoint eventPos = mouseEvent->globalPos();
+        QPoint diff = eventPos - m_mouseMovePos;
+        QPoint newPos = widgetPos + diff;
+
+        widget->move(widget->mapFromGlobal(newPos));
+
+        if (!m_isDragging) {
+          QPoint totalMovedDiff = eventPos - m_mousePressPos;
+          if (totalMovedDiff.manhattanLength() > 3) {
+            m_isDragging = true;
+          }
+        }
+
+        m_mouseMovePos = eventPos;
+      }
+    } break;
+    case QEvent::MouseButtonRelease: {
+      if (m_isDragging) {
+        m_isPressing = false;
+        m_isDragging = false;
+        event->ignore();
+        return true;
+      }
+    } break;
+  }
+
+  return QObject::eventFilter(obj, event);
+}