Add GNotification

https://bugzilla.gnome.org/show_bug.cgi?id=688492
Add gtk notification backend

https://bugzilla.gnome.org/show_bug.cgi?id=688492
GNotification: some final cleanups

Move a method from GNotificationBackend into the fdo backend (since it
was only used from here).  Remove the accessors for the already-public
(in private header) ->dbus_connect and ->application on
GNotificationBackend.

https://bugzilla.gnome.org/show_bug.cgi?id=688492
GNotification: finish documentation

Add the missing documentation and add the section to the GIO reference
docs.  Fix up a few small documentation issues.

https://bugzilla.gnome.org/show_bug.cgi?id=688492