window-actor: add new signal "effects-completed"

window-actor: add new signal "effects-completed"

When using plugins, the effects will affect the MetaWindowActor size
and position.

Add a new signal "effects-completed" wired to the corresponding
MetaWindowActor which is emitted when all effects are completed so that
derived objects can be notified when all effects are completed and use
the actual size and position.

(cherry picked from commit 85bbd82ae847eed0bba943c119a356d9493f7da2)

Bug: https://gitlab.gnome.org/GNOME/mutter/issues/104
Origin: upstream, 3.28.3, commit:e4661d7870c4ae939c72371b5c4b46d5390bfeac

Gbp-Pq: Name window-actor-add-new-signal-effects-completed.patch