[REQ] folder content image override

[clomaka]

Currently, clicking the "Display Folder Content Recursively" button will
-show a folder icon for directories having no album/cover art
-show the album/cover art if they all are the same
-show the first four album/cover arts if there's more than one.

I would like to be able to be able to specify an image to be used per folder, be it having to have a specific filename or format, whatever