NotificationDaemon: remove source if notification sender is removed from DBus

We don't want sources that are no longer associated with a running application
to stick around in the message tray.

Message tray sources were removed when the associated application’s state
changed to Shell.AppState.STOPPED . This caused sources for applications
that were still running, but did not have any open windows to be removed.
Instead, we should use the notification’s sender removal from DBus as an
indicator for when to remove the associated source from the message tray.

https://bugzilla.gnome.org/show_bug.cgi?id=645764