window-actor: Keep in compositor's window list until destroyed

When a window is destroyed, the corresponding actor may still be
kept around for the destroy effect. But as the actor is removed
from the compositor's stack list immediately, the compositor will
always stack it above "valid" window actors - this is not what we
want, so only update the compositor's list when the actor is
actually destroyed.

https://bugzilla.gnome.org/show_bug.cgi?id=735927
window-actor: Consider needs_destroy in is_destroyed()

According to the documentation, the method returns "whether the X window
that the actor was displaying has been destroyed" - that is very much
true when we delay the actual actor destruction for a destroy animation,
so update the method accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=735927
window-actor: Skip frame-sync when the corresponding window is gone

https://bugzilla.gnome.org/show_bug.cgi?id=735927