Services API: Expand the sorting in Video/GetVideoList

You could already sort by added and released this just expands on that allowing
the list to be sorted by other fields.

This is the full list of sortable fields some make more sense than others:-
title, director, studio, plot, rating, year, releasedate, userrating, length,
playcount, filename, hash, showlevel, coverfile, inetref, collectionref, homepage,
childid, browse, watched, playcommand, category, intid, trailer, screenshot,
banner, fanart, subtitle, tagline, season, episode, host, insertdate, processed,
contenttype