Differently from the fd.o notifications, Gtk notifications do not
have a mechanism to update themselves. Instead, when a new
notification is received for an ID already known to the notification
daemon, the old notification is dismissed and a replaced with a new
one.
Currently though, there is no way to distinguish a notification that
was dismissed because of an user interaction, or because it was
replaced. That is an useful piece of information, so add a new value
to the NotificationDestroyedReason enum to account for it.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/258
a98ed08a54