Fall back to the legacy format-specific scanners for WMA, FLAC and Musepack,
as ffmpeg doesn't report metadata for FLAC nor Musepack and has bugs with
some WMA variants.
Use ffmpeg's (starting with SVN 20090301) new metadata API to extract
metadata from virtually all formats supported by ffmpeg. That will allow
to get rid of some dependencies and some open-coded routines that aren't
maintainable.
