The extended attribute filter allows new joins in a fetch (content-list), but the sorting possibilities is limited as they are predefined.

This patch makes it possible to add new columns to the fetch query, and sort the result on them by using extended attribute filter. If the extended attribute filter is not used only the predefined sorting types are allowed.

Tested on 3.9 stable rev. 19706. Patch applied successfully to trunk rev. 19707.