fetchart: Extract embedded art

Is it possible to fetchart plugin to, if a file has an embedded cover, just copy it to a cover file instead of downloading?

Start by reading the fetchart plugin entry point and tracing how it handles files with embedded cover art versus downloading artwork. Test the behavior with a file containing an embedded cover, and consider the work done when that art is copied to the expected cover file instead of downloaded.

Written by the indexing model from the issue text.