clients: Add a widget_get_wl_subsurface

Adds a simple accessor for the wl_subsurface for widgets created with
window_add_subsurface.
diff --git a/clients/window.h b/clients/window.h
index 777bfbc..cf8fc6c 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -345,6 +345,9 @@
 struct wl_surface *
 window_get_wl_surface(struct window *window);
 
+struct wl_subsurface *
+widget_get_wl_subsurface(struct widget *widget);
+
 enum window_buffer_type {
 	WINDOW_BUFFER_TYPE_EGL_WINDOW,
 	WINDOW_BUFFER_TYPE_SHM,