When a daemon like gnome-settings-daemon has multiple plugins, if a plugin uses
notify_set_app_name() then subsequent notifications get the wrong
'Application name' in the gnome-shell persistent message tray.
This new function is per-notification and is thus allows us to set a custom
application name without overwriting the application name stored by notify_init().
https://bugzilla.gnome.org/show_bug.cgi?id=648947