LiveForm should track "saved" and form validity

[Jonathan Jacobs]

A common query is testing whether a form's contents have all passed validation, this can easily be tracked in "setInvalid" and "setValid"; although sometimes one also wants to know whether the form has been successfully submitted before (testing whether the data on the server is valid, even if the unsaved form is invalid.) e.g. Testing if it is safe to reveal more UI.