CelView: Add dragging for mouse

This patch adds dragging for mouse, so by pressing shift dragging mode
is being changed to scroll hand drag, basically it allows to navigate on
the image without pressing scroll bars.

source/celview.cpp

@@ -22,16 +22,43 @@ CelScene::CelScene(QWidget *v)
 
 void CelScene::mousePressEvent(QGraphicsSceneMouseEvent *event)
 {
-    if (event->button() != Qt::LeftButton) {
-        return;
-    }
+    switch (event->button()) {
+    case Qt::LeftButton: {
+        // return if currently set drag mode is scroll hand drag
+        if (views()[0]->dragMode() == QGraphicsView::ScrollHandDrag)
+            return;
 
-    int x = event->scenePos().x();
-    int y = event->scenePos().y();
+        int x = event->scenePos().x();
+        int y = event->scenePos().y();
 
-    qDebug() << "Clicked: " << x << "," << y;
+        qDebug() << "Clicked: " << x << "," << y;
 
-    emit this->framePixelClicked(x, y);
+        emit this->framePixelClicked(x, y);
+        break;
+    }
+    case Qt::MiddleButton: {
+        views()[0]->setDragMode(QGraphicsView::ScrollHandDrag);
+
+        // after middle button has been pressed - send the mouse press event to QGraphicsView that holds this scene, since
+        // it will toggle on dragging on ScrollHandDrag
+        QMouseEvent eventGraphicsView(QEvent::MouseButtonPress, event->scenePos(), event->scenePos(), Qt::LeftButton, Qt::LeftButton, Qt::NoModifier);
+        QApplication::sendEvent(views()[0]->viewport(), &eventGraphicsView);
+        break;
+    }
+    }
+}
+
+void CelScene::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
+{
+    switch (event->button()) {
+    case Qt::LeftButton: {
+        break;
+    }
+    case Qt::MiddleButton: {
+        views()[0]->setDragMode(QGraphicsView::NoDrag);
+        break;
+    }
+    }
 }
 
 void CelScene::dragEnterEvent(QGraphicsSceneDragDropEvent *event)

source/celview.h

@@ -28,6 +28,7 @@ class CelScene : public QGraphicsScene {
     CelScene(QWidget *view);
 
 private slots:
+    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event);
     void mousePressEvent(QGraphicsSceneMouseEvent *event);
     void dragEnterEvent(QGraphicsSceneDragDropEvent *event);
     void dragMoveEvent(QGraphicsSceneDragDropEvent *event);