Actions might get detached sometime during event processing,
at a time that the stage did already prepare an emission chain
holding references to the actions and actors that need to handle
events. This means actions might become detached, but still handle
the incoming event, or possible crossing events generated in-place
when the actor becomes unparented.
Avoid this situation, by skipping event handling on actions that
went detached, we will just instruct to continue event processing.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3988>
fe9fff2729