Add API to retrieve the interface name of a wl_resource

Signed-off-by: Giulio Camuffo <giulio.camuffo@kdab.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>