LibGUI: Make Action::set_text() update any associated menu items

Now you can change the text of an action and it will actually show
up in the menu. :^)

Userland/Libraries/LibGUI/MenuItem.h

@@ -50,6 +50,8 @@ public:
     void set_menu_id(Badge<Menu>, unsigned menu_id);
     void set_identifier(Badge<Menu>, unsigned identifier);
 
+    void update_from_action(Badge<Action>) { update_window_server(); }
+
 private:
     void update_window_server();