Slight changes to QChangeArbiter

QChangerArbiter now takes an int instead of a QChangeFlag when
registering an observer.
Completed unregister methods and changes are only forwarded to
observer that monitor that type of change.

Change-Id: I67600eaa2803e0a0b08f700a0cc789e64e807b4e
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>