developer UI should not allow some picos to be deleted

Quoting from our documentation, "Currently, the UI does not allow you to delete a pico which has children (until you first delete all of its children, etc. recursively)."

We need to make this so.

While the engine can just up and delete an entire sub-tree of picos, we don't want this available from the UI. Such a thing should only be done programmatically, and is in the province of Wrangler.

What's asked for here is a simple UI change to not present the "delete" link for a pico that has any children.