Done Jan 30.

Claire and France have begun marking up several documents without starting with the categories from previous documents. You can start a new document using categories from a previous doc, but once you've started marking up a document, you can't then go back and import categories from another. This makes sense as a feature, and it has a real use-case right now, so I'll add it for the next version. In the case of collisions (imported category has same id as existing category), the import of that category should be skipped.