R1:40680e459f44

Add a basic multipage form

Summary:
Ref T2232. Very busy day on IRC so I feel like I've made 20 minutes of progress in 1-minute spurts here, but this adds the basics for a form that can have multiple pages and automatically handle pagination and reading to/from the request, objects and responses.

The UIExample is reasonably instructive. Basically, you make a form, add pages to the form, and add controls to the pages. The core flow control looks like this:

if ($request->isFormPost()) {
$form…