theme: add ".appmenu" class to the appmenu button

So it does gtk headerbar, so mutter should do.

src/ui/theme.c

@@ -712,6 +712,8 @@ get_class_from_button_type (MetaButtonType type)
       return "maximize";
     case META_BUTTON_TYPE_MINIMIZE:
       return "minimize";
+    case META_BUTTON_TYPE_APPMENU:
+      return "appmenu";
     default:
       return NULL;
     }