window, desktop-shell: deal with output unplug on client side

when output is removed, weston-desktop-shell should destroy panel
and background surface on destroyed output.

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
diff --git a/clients/window.h b/clients/window.h
index 461ec1e..ff7c87b 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -104,6 +104,9 @@
 void
 display_set_global_handler(struct display *display,
 			   display_global_handler_t handler);
+void
+display_set_global_handler_remove(struct display *display,
+			   display_global_handler_t remove_handler);
 void *
 display_bind(struct display *display, uint32_t name,
 	     const struct wl_interface *interface, uint32_t version);