Clutter actors unset their parent on dispose, after emitting the ::destroy
signal, however this could cause ::parent-set signal 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 if the actor is being
destroyed.
Update the actor-destroy test to verify this behavior.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/769
f376a318ba