We want to minimize focus stealing from the user. If a non-urgent notification
comes in while the user is interacting with the tray, we add it to the tray
and only show it after the user is done interacting with the tray. If an
urgent notification comes in while the user is interacting with the tray,
we hide the tray and show the urgent notification.
https://bugzilla.gnome.org/show_bug.cgi?id=636838