Application examples: Add an AppMenu example.

	* examples/Makefile.am:
	* examples/book/application/app_menu/: This is based on the simple
	example, without the document handling, and is meant to show the
	use of set_app_menu(), using Gio::Menu, Gio::SimpleAction, and the
	base Gio::ActionMap API.
	However, I just get lots of these errors:
	GLib-GIO-CRITICAL **: g_dbus_connection_register_object: assertion `G_IS_DBUS_CONNECTION (connection)' failed