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>

unstable/xdg-shell/xdg-shell-unstable-v6.xml

@@ -171,7 +171,7 @@
 	xdg_popup is and how it is used.
       </description>
       <arg name="id" type="new_id" interface="zxdg_popup_v6"/>
-      <arg name="parent" type="object" interface="wl_surface"/>
+      <arg name="parent" type="object" interface="zxdg_surface_v6"/>
       <arg name="seat" type="object" interface="wl_seat" summary="the wl_seat of the user event"/>
       <arg name="serial" type="uint" summary="the serial of the user event"/>
       <arg name="x" type="int"/>