Add meta_window_get_wm_class

Public function to retrieve the cached WM_CLASS, used in gnome-shell
for tracking applications.

src/core/window.c

@@ -8687,6 +8687,21 @@ meta_window_get_description (MetaWindow *window)
   return window->desc;
 }
 
+/**
+ * meta_window_get_wm_class:
+ * @window: a #MetaWindow
+ * 
+ * Return the current value of the WM_CLASS X property.
+ */
+const char *
+meta_window_get_wm_class (MetaWindow *window)
+{
+  if (!window)
+    return NULL;
+
+  return window->res_class;
+}
+
 /**
  * meta_window_get_compositor_private:
  * @window: a #MetaWindow