Fix ordering of aggregates that are based on a 'directory' property

When defining an aggregate of type Terms (in a page provider or the default search) with the order by property set to term asc or term desc for a schema property of type directory, aggregates are sorted on the id of the directory entry in the JSF UI, which does not always correspond to the alphabetic sorting of the directory entries' label; the directory entries' label being what is displayed in the UI, sorting on the id does not actually make sense for a user's point of view.