I wrote a UI builder for OS/2 PM that targeted PM or Windows. It also had supporting libraries and code generators that let you pick many standard formatting and validation functions as well as hook in your own. That was around 1990.
Why are we still only specifying the layout in this day and age? We should be able to spec most of an application 'in the UI builder' and only add the data processing parts, even the network access or persistence should be wired for us.