An input only grab is a ClutterGrab on the stage that doesn't have an
explicit actor associated with it. This is useful for cases where event
should be captured as if focus was stolen to some mysterious place that
doesn't have anything in the scene graph that represents it.
Internally, it's implemented using a 0x0 sized actor attached directly
to the stage, and a clutter action that consumes the events. An
input-only grab takes a handler, user data and a destroy function for
the user data. These are handed to the ClutterAction, which handles the
actual event handling.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2628>
Since we want these accessed from bindings this must be a boxed
type. This has the side effect of making ClutterGrab a refcounted
object, since we want to avoid JS from pointing to freed memory
and maybe causing crashes if misusing the object after dismiss.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2099>