I should be able to delete a Model Type

Summary: Currently, it just can't be done -- the back end forbids it.

The only complication here is that you should only be allowed to delete a Model Type iff there are no Properties using it. Or (better) we should do the same thing we do with Property deletion, put up a warning if the Model Type is being used (and the Properties and Things that depend on it), and recursively delete if requested.

In fact, there's an even deeper case here: we currently do allow you to delete a Model that a Type depends on, which is pretty broken. (I shouldn't be able to just delete a Model that a Type depends upon) That should follow the same procedure, and is the straw that breaks the camel's back: we really need a general concept of "dependencies", which propagates up to the delete command in the UI, and fixes things up recursively if approved. (This recursion should apply to nested Model Types!)