Show embedded thumbnails if available

[microraptor]

Some formats like MKV and MP3 can save embedded thumbnails and album arts. It would be great, if those were used whenever available instead of a frame of the video for the playlist view.

Here is how ffmpeg can extract them:
https://superuser.com/questions/1328204/extract-video-cover-thumbnail-from-file-with-embedded-cover-using-ffmpeg
https://stackoverflow.com/questions/13592709/retrieve-album-art-using-ffmpeg