Introduce keyboard grabbing protocol for Xwayland

This patch introduces a new protocol for grabbing the keyboard from
Xwayland.

This is needed for X11 applications that map an override redirect window
(thus not focused by the window manager) and issue an active grab on the
keyboard to capture all keyboard events.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>