Add a feature that will allow editors to associate artwork with a specific tracks within a release.

Eventually support should be added to Picard to tag tracks with the appropriate artwork, and not just the first piece of art that's happens to be listed as a cover.

Examples of releases that would benefit from this:

• TBD (feel free to edit with examples)

Note: This ticket should share UX with/needs to be considered together with per-medium artwork: https://tickets.metabrainz.org/browse/MBS-5449