LibGUI: Add a GToolBar class that can be populated with GActions.

The same action can be added to both a menu and a toolbar.
Use this to put a toolbar into FileManager. This is pretty neat. :^)

LibGUI/GMenu.cpp

@@ -29,7 +29,7 @@ GMenu::~GMenu()
     unrealize_menu();
 }
 
-void GMenu::add_action(OwnPtr<GAction>&& action)
+void GMenu::add_action(RetainPtr<GAction>&& action)
 {
     m_items.append(make<GMenuItem>(move(action)));
 }