QmlDesigner.formEditor: use pure virtuel instead of assert

Asserting because of unexpected mouse events is evil.

Task-number: QTCREATORBUG-6042
Change-Id: Idc21b1cb6bfebead4eab80f6526235164b3f8d54
Reviewed-on: http://codereview.qt-project.org/5090
Reviewed-by: hjk <qthjk@ovi.com>

src/plugins/qmldesigner/components/formeditor/movetool.cpp

@@ -203,6 +203,16 @@ void MoveTool::keyReleaseEvent(QKeyEvent *keyEvent)
     }
 }
 
+void  MoveTool::dragLeaveEvent(QGraphicsSceneDragDropEvent * /*event*/)
+{
+
+}
+
+void  MoveTool::dragMoveEvent(QGraphicsSceneDragDropEvent * /*event*/)
+{
+
+}
+
 void MoveTool::mouseReleaseEvent(const QList<QGraphicsItem*> &itemList,
                                  QGraphicsSceneMouseEvent *event)
 {