Handle cover art for multi-disc albums stored in seperate folders

[arthurl]

Issue description

I typically store multi-disc albums using the following directory structure:

$albumname/CD01/01 $disc01-track01_name
$albumname/CD01/02 $disc01-track02_name
$albumname/CD01/...
$albumname/CD02/01 $disc02-track01_name
$albumname/CD02/...
$albumname/cover.jpg

In this case, cover.jpg is not picked up as the album cover, even though LMS correctly recognises all the directories as a single album.

Q: How are covers currently handled?

I’ve tried looking at the code, but I’m a bit confused. When is cover art retrieved using Grabber::getFromRelease vs Grabber::getFromTrack (in src/cover/CoverArtGrabber.cpp)? Isn’t cover art displayed only for entire albums, and not for the individual track?

[epoupon]

Actually, LMS tries to get the release cover from the first scanned track of this release.
If the cover is embedded in the audio file, that's done.
If it is not embedded in the audio file, it will try to find a cover file in the directory where the file is stored.
Here I would say it looks in the directory 'CD01'.
Not sure if looking for a cover file in the parent directory makes sense, but I guess we could do it.