backends/x11: Implement ClutterSeat::touch-mode for the X11 backend

This only checks touchscreen availability as we have no access to
tablet-mode switch events as we do on the native backend.

Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1242

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1278


(cherry picked from commit 38bbd959)