clutter/actor: Don't emit signals and property notifications after 'destroy'
Clutter actors unset their parent in dispose, after emitting the ::destroy signal, however this could cause the ::parent-set and ::notify signals emission. Since we assume that after the destruction has been completed the actor isn't valid anymore, and that during the destroy phase we do all the signal / source disconnections, this might create unwanted behaviors, as in the signal callbacks we always assume that the actor isn't in disposed yet.
To avoid this, don't emit ::parent-set signal or don't notify if it's being destroyed.
Update the actor-destroy test to verify this behavior.
This Fixes errors we're getting when an idle or a later is added on
::parent-set (like here) and that will try to access to the actor that has just been destroyed, as if this happens after 'destroy' it will be just kept running.