Folder based artwork naming improvements

[timusus]

Fixes #169

The regex pattern for detecting album art files has been expanded to include common filename variations that were previously not recognized:

• albumart.jpg/png (iTunes/Windows Media Player standard)
• front.jpg/png (front cover artwork)
• artwork.jpg/png (generic artwork files)

These additions complement the existing patterns (folder, cover, album) and maintain support for hidden files (those starting with a dot).

The pattern now matches:

• folder.jpg, cover.jpg, album.jpg (existing)
• albumart.jpg, albumartsmall.jpg (new)
• front.jpg, frontcover.jpg (new)
• artwork.jpg (new)
• All with optional leading dot for hidden files