notificationDaemon: Fix urgency hint

We currently mark notifications as urgent which merely contain the
'urgent' property, even when set to false. Look at the actual value
instead.

https://bugzilla.gnome.org/show_bug.cgi?id=710596
notificationDaemon: Fix custom icons

The 'icon' property contains a serialized GIcon, so we need to
deserialize it when setting the icon.

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