webadmin: Changed editor prohibition reason mechanism

The tooltip displayed over disabled editor widgets (change prohibition
reason) is now updated upon call to setChangeProhibitionReason() as
well as to setIsChangeable(). This means that the correct tooltip will
be shown even if setChangeProhibitionReason() is called after
setIsChangeable(false), which hasn't been the case up 'til now.

Change-Id: Ifb76b2348922bfebf62c54da4ee2a8ca45ebef31
Signed-off-by: Lior Vernia <lvernia@redhat.com>