Compilations detection should be more comprehensive

Exaile 0.3.0a3, rev 2152

At the moment, an album is marked compilation as soon as one song has different artists than the rest of the album. It becomes worrying when for instance, all songs have "Foo" as an artist, and one song also has "Bar" as an artist (ie. as two artist tags, one "Foo" and one "Bar"). In such cases, the album should be marked as a "Foo" album in the collection view (but a search on "Bar" should also return it :D If the view is set to 'artist', maybe it can be done by adding a "From other artists' albums" line in which songs for which "Bar" is a guest could be displayed).

There are a lot of albums where an artist has one or two songs with featuring guests, they shouldn't all end up as compilations.

Also, when one searches the collection, in artist mode, with a search term that has no matches, then "Various Artists" is shown with 0 songs inside it, while nothing should be showed at all.

These bugs should ideally be fixed for 0.3.0 :)