Comment 1 for bug 462512

Redoes the local cover art search, use os.listdir() rather than trying os.path.isfile() for all possible filenames which is not practical once we consider mixed cases. Keeps the check for is a file and the correct access requirements. Note this preserves the existing priorities (the order recorded in the preferences).

Also note that I was seeing iterate_candidates sometimes called with None as the track_path, so I added a check for this.

Developed and test on Linux with Python 2.6