Make it possible to remove existing tags without clearing all tags

Right now, it seems the only way to remove a tag that's already in a file's tags is to use the clear existing tags option. It would be really nice to be able to remove individual tags without having to use that option.

e.g. I generally want to keep the existing tags, but I do want to remove genre. If I could do something like $delete(genre), that would mean I wouldn't have to use another program to remove them.

Also, if you right click on a new value, there is an option to remove it. If the tag already exists in the file, that option isn't there.