Two-part transitions

[jace]

Just like properties have two aspects, the getter and the setter, transitions need two parts:

1. The GET aspect of the transition returns a form indicating what sort of response it expects from the client.
2. The POST aspect of the transition accepts a filled out form and if valid, performs the transition. If invalid, it returns the form again with a list of errors.

Having a two-part transition will make it possible to hook them up directly with views. The current approach requires a separate GET view per transition.

[jace]

One way to implement this:

1. Convert transitions into callable class instances. The default call handler does the equivalent of a POST operation.
2. Add a .form (or similar) decorator that provides a form, the equivalent of a GET request.
3. Add a .validate decorator that validates the POSTed form and returns a boolean.

A view handler can use these three methods to perform a transition without needing to know anything about the data expected by the transition. It simply passes the form around.