LibGUI: Add a simple "recently open files" feature

This feature allows any application to easily install an automatically
updating list of recently open files in a GUI::Menu.

There are three main pieces to this mechanism:

- GUI::Application::set_config_domain(domain): This must be called
  before using the recent files feature. It informs the Application
  object about which config domain to find the relevant RecentFiles
  list under.

- GUI::Menu::add_recently_open_files(callback): This inserts the list
  in a menu. A callback must be provided to handle actually opening
  the recent file in some application-specific way.

- GUI::Application::set_most_recently_open_file(path): This updates
  the list of recently open files, both in the configuration files,
  and in the GUI menu.

Userland/Libraries/LibGUI/Menu.h

@@ -48,6 +48,8 @@ public:
     Menu& add_submenu(DeprecatedString name);
     void remove_all_actions();
 
+    ErrorOr<void> add_recent_files_list(Function<void(Action&)>);
+
     void popup(Gfx::IntPoint screen_position, RefPtr<Action> const& default_action = nullptr, Gfx::IntRect const& button_rect = {});
     void dismiss();
 
@@ -78,6 +80,8 @@ private:
     NonnullOwnPtrVector<MenuItem> m_items;
     WeakPtr<Action> m_current_default_action;
     bool m_visible { false };
+
+    Function<void(Action&)> m_recent_files_callback;
 };
 
 }