Tag merge improvements

This change will allow a user to split tags and is designed to work in conjunction with the tag merging feature.  I have some tags that were imported with songs that are compound tags, such as "Hard Rock & Metal."  This will allow you to edit a tag like that and in bulk copy all mappings to a new tag while keeping the existing links in place.  In my usage, I would merge the exiting tag in to "Hard Rock" and then split it into an existing "Metal" tag, thus eliminating the compound tag.  All songs would then be tagged with "Hard Rock" and "Metal" tags.