include basic tags in tagdiff view, even if no value is set.

The new view of tags is great, but if a field is blank there is no way to edit it because it doesn't show up. I propose that a small number of fields be always shown, which should basically be the list of fields that were shown in the old interface: title, artist, album, track number

alternatively, an option somewhere to set a list of tags that will always be available.