Display multiple labels/catalog numbers where applicable

Currently, beets only shows one catalog number, which is probably correct for a majority of releases. Some releases, however, have more than one label and more thus more than one catalog number. In a lot of cases you might only have one of these visible where you got the album from, so it would be useful to be able to see all of them during an import.

I know it's a goal to keep the importer output short, so it might just be that this would make it too long. In that case maybe this would be more suitable for a plugin?

Start by tracing the importer output that currently displays a single label and catalog number, then review how plugin-based importer output is supported. Done means deciding whether the core importer or a plugin should expose all applicable labels and catalog numbers without making normal import output excessively long.

Written by the indexing model from the issue text.