compositor: Detach window and actor both ways on actor dispose
This is currently deferred to the window being unmanaged, however the dependency of events is too indirect to consider these things always happen one after the other.
Fixes possible warnings like:
(mutter:872101): GLib-GObject-WARNING **: 15:29:27.943: invalid unclassed pointer in cast to 'MetaWindowActor'
Seen on shutdown, a reliable reproducer is gnome-settings-daemon test suite.