notificationDaemon: Keep source alive when replacing notification

notificationDaemon: Keep source alive when replacing notification

When a GNotification has the same ID as a previous one, it should
be shown as a new notification after withdrawing the old one.
However for this to work, we must not destroy the corresponding
source if withdrawing the old notification lets the notification
count drop to zero, so make sure the source is kept alive until
the replace operation has completed.

https://bugzilla.gnome.org/show_bug.cgi?id=775149