Handle id3v23 tags in existing files in a better way.

For mp3 files saved with ID3v23 tags (which is common as  Windows does not support ID3v24) many multi-value tags are flattened. This commit fixes the situation where this causes mismatches between flattened original tags and unflattened MB data when these files are reread.

It does so by improving the comparisons between flattened Original and unflattened New values. This was being complicated by multi-value tags being sorted in the UI but not the saved tags - which goes against the discussions I have had with luks and bitmap where we agreed to make the UI as representative of what is stored in the files as possible. Consequently I have removed the sorting from the UI and provided a Sort plugin to replace it. At some point this could be moved into the Tag options, however as an interim I have provided a plugin instead.