QmlDesigner: Fix slowness for many items

The information change notifier provided no hint which infotmation changed.
So the property editor has updated the anchors for all information changes.
Now there is a hint and the update is only happen for a anchor change.

Change-Id: I3b6d7546f43bce4c08757662e7af32604a8db56f
Reviewed-on: http://codereview.qt.nokia.com/811


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>