App_and_win_menus example: Add a menu item that sends a Gio::Notification

* examples/book/application/app_and_win_menus/exampleapplication.[h|cc]:
Add on_action_print(). Add actions and menu items for a Gio::Notification.
* examples/book/application/app_and_win_menus/examplewindow.[h|cc]:
Add on_action_send_notification() and on_action_withdraw_notification().