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!
It's deliberately case-sensitive as it is (we want them returned in lowercase anyway), but I don't have anything against using xdigit, sure.