environment: Ignore animation events after shutdown

environment: Ignore animation events after shutdown

As soon as the shutdown starts, any animation event does not make sense,
although during this phase the clutter timelines may still proceed and
potentially calling invalid code in their callbacks.

Prevent this from happening by disconnecting from any timeline signal
we are connected to as soon as the `shutdown` signal is emitted

Co-Authored-By: Florian Müllner <fmuellner@gnome.org>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3996>