`g_notification_backend_new_default()` adds a reference on
`backend->dbus_connection` (if non-`NULL`), but nothing ever unreffed
that.

Fix that by adding a dispose method.

In practice this is not really a problem, because the notification
backend is held alive by a `GApplication`, which lives as long as the
process. It’ll be a problem if someone is to ever add unit tests for
`GNotificationBackend`s though. So let’s fix it.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>