Users of Clutter grabs may listen for notify::revoked changes in
order to know that their grab is no longer in charge of event
propagation, without the use of crossing events.
Since a ClutterGrab may stay in the stack and regain effects,
this notification also happens the other way around.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3420>