Improved musicbrainz matching from tags

Currently various software (e.g. Picard) drops MBIDs into ID3v2.3 tags with various random separators.  I've seen semicolons, semicolons+space, slashes etc.  XBMC doesn't detect this as yet, even though MBIDs take an obvious, known form.

This commit fixes it by using a regexp to detect any MBIDs in the tag regardless of separator.

@night199uk please review and do a few tests - thanks!
Maybe [[:xdigit:]] would be a bit more self-documenting than [0-9a-f]?  Also save us the trouble should we switch to case-sensitive regex in the future for whatever reason.