Will be used to signal test cases. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1993>
While at it, make it a GObject so one can add signals to it. Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1716 Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1795>