Add cpe merge to combine metadata from different tools

[ssssam]

My current requirement is: I need the 'tracker.url' field to be promoted to the 'url' field in the never-listened use case, so that I can create an .m3u playlist from the result.

This tool is crucial for Calliope to be useful, we need to be able to combine metadata from multiple sources to produce the 'best' output. For example if Tracker and Musicbrainz have different options about the 'artist' for a track, the Musicbrainz version should be preferred.