As soon as any event of a sequence is handles/stopped during emission,
all actors and actions that would have gotten to see it afterwards have
a big problem: If that event was a TOUCH_END event, the actor/action is
forever going to think that this touch is still active.
For ClutterActions, we're going to handle this by introducing a way to
send them a notification when stuff like this happens.
As a baby step towards all that, make event emission exclusive to actors
as soon as any actor stopped an event.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2342>
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