When I remove a Property from a Model, I should be able to delete it

Summary: Frequently, the reason to remove a Property from a Model is that I decide I didn't want quite that Property at all. Intuitively, I want to delete that Property and free up the namespace.

The motivating example is from Dad's Spice Space. He created a "Brand" Property with the wrong Type. He removed it from the Model, and tried to recreate it with the right Type, and was surprised that the system told him that there was already a Property of that Type still. He found this confusing, and I think it's a usability bug.

At the least, we should detect when we are removing a Property that has no other usage, and offer to delete it. Quite likely the right course of action is that, whenever we remove a Property from something, we should check whether there are any other uses; if not, we should delete the Property outright. A Property that is not on anything isn't very useful. (But this should only happen if the Property is local to this Space, not inherited from an App.)