xwayland: Fix a race condition when setting up input devices

The compositor may send us wl_seat and its capabilities before sending
e.g. relative_pointer_manager or pointer_gesture interfaces. This would
result in devices being created in capabilities handler, but listeners
not, because the interfaces weren't available at the time. So we
manually attempt to setup listeners again.

Signed-off-by: Povilas Kanapickas <povilas@radix.lt>