notificationDaemon: Save notifications on source destruction

While the existing comment is correct in that a source's notifications
will be destroyed first, the code takes a shortcut which prevents the
Source::count-updated signal from being emitted. Given that the purpose
of the signal is to keep notification counters up-to-date which is
pointless when the source is about to be destroyed, the shortcut makes
sense; just save notifications explicitly in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=710596