This adds the actual input capturing rerouting that takes events and
first hands them to the input capture session, would it be active.
Events are right now not actually processed in any way, but will
eventually be passed to a libei client using libeis.
A key binding for allowing cancelling the capture session is added
(defaults to <Super><Shift>Escape) to avoid getting stuck in case the client
doesn't even terminate the session.
The added test case makes sure that the pointer moves again after
pressing the keybinding.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2628>
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