I should be able to enhance a Type Model in my Child Space

(User Story, Closed -> Fixed, Priority: Critical, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Apps, Data Model
Summary: In the current design of Apps, Type Models are rigid: you can't modify them in your child Space. That's unfortunate.
The reason is the interaction of the moving parts: the Property is in the App, the Property depends on the Type, and the Type depends on the Type Model. So there's no obvious place in there to let me override the Type Model's structure in my child Space.
This isn't a killer feature, so it's not high priority, but it would be more in keeping with Querki to allow me to add fields to the Type Model in my Space. The only question is, how do we make that work within the architecture, without making things excessively brittle? This needs brainstorming and clever ideas.