Add support for several artists of the track

[timocov]

Currently it seems that pbl uses the only first artist from the list of artists, which makes impossible to filter tracks where are several artists (featuring tracks). I think it'd be good improvements to allow to apply filters for all artists (and at the same time for the main artist only, for example by adding the new filter like "artists filter" and rename "artist filter" to "main artist filter" or so).