LibGUI: Give GTextEditor a context menu.

Now GTextEditor manages its own editing actions (cut/copy/paste/etc) and
will show a context menu containing them when requested.

Apps that want to put a GTextEditor's actions in its menu can get to the
actions via public getters. :^)

LibGUI/GMenu.cpp

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