wayland: add a release request on wl_seat

This is required if we want to correctly remove a wl_seat compositor-side. A
wl_seat is announced as a global object, then it is bound by the client. When
the compositor wants to remove the seat, it shall announce the global removal of
the object. The client can then call the release request on the wl_seat (which
means I won't use that object anymore).

Acked-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>