Implement a better discrimimaton logic between multiple cover candidates
If we end up using a cover file in an album folder, the first file is used, regardless of its pertinence.
We need to improve this to avoid using useless images, or potentially less relevant ones. For instance in my tests, usually the front cover is better. Maybe we could aim at using something that approaches the album title if available.
We could also try to detect if a cover looks too black, the code could then be shared with the thumbnailer to avoid using black frames as video thumbnails (#151)