Add unmapped field

[andrewmckaskill]

I would like to be able to add a field to UI builder without requiring a backing field on the model.

This would be used for labels, or for custom editors that used their own APIs to retrieve data about the entity.

Examples:

1. A "name" field that allowed formatting of the Name for display purposes, composed of the "FirstName" and "LastName" fields.
2. An empty field that uses a custom editor to retrieve and display a list of audit logs for a given entity.