notificationDaemon: Unpack button label

deep_unpack() doesn't unpack as deeply as one might hope ...

https://bugzilla.gnome.org/show_bug.cgi?id=710596
notificationDaemon: Fix button parameter name

Gio ended up using 'target' rather than 'action-target'.

https://bugzilla.gnome.org/show_bug.cgi?id=710596