Since property change signals can be emitted in any order we have to rely
on selection changed signals to guarantee a valid state.

I use the isInValidState which is only true after all properties have been
setup correctly after a selection change for this.

I added the signal selectionChanged for this.

Task-number: QTCREATORBUG-12642
Task-number: QTCREATORBUG-12506
Change-Id: Idf3bbe8236ece541ca267efb1bd68e0737c99716
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>