External URL's Display Name should be prompted as "Label"

Summary: Kate got to editing an External Link, and the "Display Name" is just plain confusing. It should show up as "Label" or something like that in the Editor.

This is actually the tip of a big iceberg. The problem, of course, is that there is no way to relabel the use of a Property in a Model. That is, the Prompt is a characteristic of the Property, but that's rarely useful -- what I want is to be able to change the Prompt of how this Property is shown in the context of a specific Model that's using it. Which in turn implies that there ought to be a layer of metadata between Property and its usage. That's not easy to add.

Can we create a sort of meta-Property, that wraps around a real Property and alters certain meta-Properties? Alternatively, could we derive child Properties based on parent ones, that tweak settings? The latter seems unlikely to work -- our queries assume specific Properties, not Property-and-its-children. The wrapper idea isn't crazy, but isn't easy.

The clumsy, but possibly best, approach would be to introduce a new Property which is specifically for this sort of metadata. The new PropertyMetadata Property would be set on the Model. It would contain a map of Property -> Metadata pairs. That Metadata, in turn, could contain a Prompt property, that is the override Prompt for this Property.

That's all a bit fugly, and requires some care in order to not get horribly slow, but seems like it should work.