compositor: use new drag icon listener for setting up drag surfaces

That way we can get rid of the call to
weston_compositor_update_drag_surfaces() on shell->map().
diff --git a/src/compositor.h b/src/compositor.h
index 58bae0f..d76e99d 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -121,6 +121,8 @@
 	struct wl_listener touch_focus_listener;
 	struct wl_resource *touch_focus_resource;
 	struct wl_listener touch_focus_resource_listener;
+
+	struct wl_listener new_drag_icon_listener;
 };
 
 struct weston_shader {