Add Gio::Notification

* gio/src/application.hg: Add send_notification(), withdraw_notification().
* gio/src/notification.[hg|ccg]: New files.
* gio/src/filelist.am: Add notification.hg.
* gio/giomm.h: Add giomm/notification.h.
* tools/m4/convert_gio.m4: Add conversions between GNotification and
Gio::Notification. Bug #722896.