Category: support category hierarchies

[dwhieb]

The Category object should contain references or embedded data for its parent category, allowing implementations to display hierarchies of categories.

abbreviation:
  eng: v tr
name:
  eng: transitive verb
parent: # <- This is a Database Reference, so it could just be an ID instead of embedded data.
  abbreviation:
    eng: v
  name:
    eng: verb

On the parent, it would work in reverse:

abbreviation:
  eng: v
name:
  eng: verb
subcategories:
  - abbreviation:
      eng: v tr
    name:
      eng: transitive verb