environment: Extract transition completion handling into helper function

environment: Extract transition completion handling into helper function

Add _setupTransitionCompletion to centralize started/stopped signal
handling and cleanup logic. Both _easeActor and _easeAnimatableProperty
now use this helper, reducing code duplication and ensuring consistent
behavior. The signal holder is created as a child of sessionSignalHolder
for proper cleanup on session shutdown.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4082>
(cherry picked from commit 3b638993b7456565ec6f60ceb20df39302313d39)