That was backwards.

The new properties should go in model 3, not model 2, of course.
A bone can now have several items.
Items needs to be ordered so first bone can be identified clearly.
Models and items can now have parents (and children)

This also required the bones to have several items, since a child uses
its ancestor's bones (that sounds weird) if they already have one with
the same name. I'm not entirely certain that this is necessary for the
model, but we'll see. All of course in the service of implementing #7.