QmlDesigner: Update complete scene

This removes painting artefacts, since
the bounding rectangle also gets smaller when updated.

Change-Id: Ic184cadbd1dbda1952012b749eadfde0da57ef7f
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

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

@@ -702,6 +702,8 @@ void FormEditorView::instanceInformationsChanged(const QMultiHash<ModelNode, Inf
         }
     }
 
+    scene()->update();
+
     m_currentTool->formEditorItemsChanged(changedItems);
 }