QmlDesigner: Prevent item hoping for reparenting

Because of the asynchronous nature of the formeditor the item can be between
to states. This looks like the item is hoping around. To prevent this a token
is sent to the instances and back. For the time frame the painting is disabled.

Change-Id: If7e937cba8171248464ad350bb14438c020b25f9
Reviewed-on: http://codereview.qt.nokia.com/1189


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Alessandro Portale <alessandro.portale@nokia.com>