Diagram-style editor for instances of models of discrete theories

[epatters]

Create a notebook editor for instances of models of discrete double theories, in a diagrammatic style of presentation (#240):

• For each object in the model, a cell constructor to create elements of that type
• For each morphism in the model, a cell constructor to create arrows between elements of the appropriate types

The UI will end up being quite similar to the one for models even though the structure being edited is shifted one level. We should be able to reuse the object/morphism editors already implemented for editing models (and if that's not possible, the components should be refactored to make it possible!).

This editor is expected to pair with the "isomorphic" data structure in #236.