Include album art file in collections

[geigerzaehler]

Cover art file

We want to store album cover art in an external collection. (See #43 for the original request.)

Here’s the suggested documentation of the feature.

• albumart_file When true, writes album artwork as separate image files alongside the tracks in each album directory. The artwork file is placed in the same directory as the first track of the album, using the filename configured by the art_filename setting. Note that artwork files are not automatically removed when albums are deleted from the collection.

Test cases

• Album art is copied/linked for album added to a collection
• Album art is updated for album already in collection
• Album art is not added for album that does not have cover art
• Album art is not added for single tracks where album is not matched
• Album art copy works even if albums share the same directory

Implementation notes

• Align with behavior of fetchart and embedart plugins
• Use Album.artpath to locate art and Album.art_destination to determine location, See fetchart.Plugin._set_art