Make it possible to sort aggregates by title

Currently aggregate values in a search form can be sorted by count or by term. However the term is not always human readable, in particular with aggregates on Document fields.

We could change the behavior of "sort by term" to rely on a specific property, depending on the type of field:

• for Document, it would sort by title
• for Vocabulary, it would sort by label
• for User, it would sort by "firstname lastname"