Add exceptions to smart playlists

[digimezzo]

The smart playlists need an except condition: e.g. it should be possible to have rap playlist: +rap, +grime, but then exclude trap. That's not possible within the current logic.

[DisasterArea96]

Was thinking maybe a smart way you could implement this is make it possible to have the rules used to generate a playlist apply to another playlist instead of the whole library

e.g. i can create a playlist where the genre is like rap or grime, then have a playlist that uses that playlist as a source and excludes those where the genre is like trap