MediaFile::hasThumbnail() reports true for music files with art that is not cover art

[James Henstridge]

Some media formats allow for the storage of multiple pieces of artwork tagged with an image type. This includes:

 1. ID3v2's APIC artwork
 2. FLAC pictures
 3. vorbiscomment's METADATA_BLOCK_PICTURE (which is just a base64 encoded FLAC picture).

Thumbnailer will only produce a thumbnail for artwork marked as "front cover" or "other". This means that if we encountered a file with art but no "front cover" art, mediascanner might direct an application to request embedded art (which will fail), when it might be possible to successfully get online artist/album artwork.

How likely this is to affect users is unknown. I don't think we've had any bug reports about this behaviour yet, so files this would effect are either exceedingly rare or don't exist outside of test suites.