Add missing 'Arranger' to mp4 tags, and temp-fix for information loss after saving Mp4

- Added missing Arranger, which is present in ID3, Vorbis, etc.
- Change multi-value tags (e.g. Composer, Arranger) into a single-value string separated by semi-colons.  This is to prevent information loss, because all Mp4 tags saved by Picard at present discards all multi-value tags except for the first value - which is behavior that is inconsistent with the GUI (which shows all the multiple values for the tag)