Be smarter about finding album art

[graue]

Currently Pots, fyi only marks an album as having art if it finds a "cover.jpg", "folder.jpg", "cover.png", or "folder.png" image in the same directory as the first file found that belongs to the album.

If there are .jpg/.png files in the directory, but none named 'cover' or 'folder', we could take the image with the largest filesize.

It would also make sense to take the longest path prefix common to all files in the album and look there for art, e.g.

Some Album/CD1/01 - Disc One Track One.mp3
Some Album/CD1/ ... [more files] ...
Some Album/CD2/02 - Disc Two Track One.mp3
Some Album/CD2/ ... [more files] ...
Some Album/folder.jpg

Right now, the art won't be found in this case, because it's not in the same directory as the first file found.