revealer: Fully set the target state if unmapped during animation
If the revealer is told do animate and then unrealize itself, we do (correctly) stop the animation, but used to do a shortcut where we just set the target state as current.
Other things are dependent on the animation properly finishing though, like the contained widget child visibility. This may lead to inconsistent state where gtk_revealer_get_child_revealed() returns TRUE but the child widget is unmapped, or vice-versa.
Fully finish the animation here, so the child state is coherent the next time the revealer is mapped. We can also skip notifying on the property since it will be handled by gtk_revealer_set_position().