xdg-shell: Make get_popup take a xdg_surface instead of wl_surface

The reason for using wl_surface before was that xdg_popup and
xdg_surface (now xdg_toplevel) had no common interface other than
wl_surface, but since xdg_surface is now the base interface, lets use
that.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Mike Blumenkrantz <zmike@osg.samsung.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Benoit Gschwind <gschwind@gnu-log.net>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>