lastgenre: Weighted combination of track, album and artist genres

### Feature
A bit more control over genre tagging process in the LastGenre plugin would be a great option. Currently it is only possible to select the track, album or artist level of tags to use. Only one of those sources will be used following the track->album-->artist priority.

Track tags aren't as robust as album tags (tracks are more likely to have less tags than albums), but they are more specific (album-level genre tags do not necessarily apply for tracks).

I propose a "hybrid" option to have a weighted combination of tags on track, album, and artist levels. Those weights can be specified by a user (for example, track: 40%, album 60%, artist: 0%). It then will make sense to apply ```min_weight``` on the hybrid tags afterwards.

Depending on the selected weights, this may help to have a bit more specific annotations on a track level, while keeping them more consistent with the genre tags on the release level.

P.S. This is just an idea, as I haven't experimented with such weights, but I think it would be a useful feature anyways.