#1074 Add support for revised or abridged works

[Marc Kupper]

At present when a story is revised or abridged we have two ways of associating it with the original story. 1) It's filed as a variant title, hopefully with notes explaining the differences. 2) The works are put into a series.

This feature request is to add a "variant type" field to better support documenting the nature of the relationship between two titles. It could be one of variant title, translation, revision, expansion, or abridged.

This would also allow the story lengths to be different and would not trigger a warning. For example, a novella may be abridged into a novelette which normally triggers a warning for variant titles.