wayland-server: Add functions to wl_global

When using a wl_global, a server may need to retrieve the associated
wl_interface and user data.

Add a couple of convenient functions wl_global_get_interface() and
wl_global_get_user_data() for this purpose.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>