Genre Field Improvement

[ericd23]

I was wondering if it makes more sense to

• let each track have its own genre: I was updating one track's genre field direclty on file. Then beet update got the db into a weird state because it seems to populate the new genre info to the album and every other tracks in it as well;
• Use SEMICOLON_SPACE_DSV for genre: (I did see other similar discussions around it) Currently I'm using , as separator, but it's a bit hacky.