application: Withdraw system notifications on exit

This patch keeps track of the system notifications as they appear
and withdraws them when the application quits.

The HIG[0] says that "Notifications in GNOME 3 persist after they
have been initially displayed. It is therefore important to remove
notification messages that are no longer relevant to the user."

[0] https://developer.gnome.org/hig/stable/notifications.html.en

https://bugzilla.gnome.org/show_bug.cgi?id=776043