window: Add accessor method for gtk_theme_variant

This is useful information for UI associated with a window, so add
a corresponding method.

https://bugzilla.gnome.org/show_bug.cgi?id=730752

src/core/window.c

@@ -7071,6 +7071,18 @@ meta_window_get_wm_class_instance (MetaWindow *window)
   return window->res_name;
 }
 
+/**
+ * meta_window_get_gtk_theme_variant:
+ * @window: a #MetaWindow
+ *
+ * Return value: (transfer none): the theme variant or %NULL
+ **/
+const char *
+meta_window_get_gtk_theme_variant (MetaWindow *window)
+{
+  return window->gtk_theme_variant;
+}
+
 /**
  * meta_window_get_gtk_application_id:
  * @window: a #MetaWindow