Support a generic 'form post-processing' hook

[droos]

Support a generic 'form post-processing' hook that lets you do any data manipulation of the model whatsoever. This is in lieu of supporting XForm actions like <setvalue>.

The idea is that based on the answers to various questions, we may need to manipulate the data in the model in complicated ways (e.g., if answer to q3 is 'yes', use value from q1, else use value from q2). This is not so much an issue for apps that always submit their data to a server. The post-processing can be done on the server. For self-contained apps, however, that rarely sync with a server, it is important to be able to clean up the data and keep it consistent and usable for future invocations of a form.

This hook would be a very simple interface in which you gain access to the mode, and you could manipulate it in any way you see fit. It should probably be executed before the item-specific post-processing handlers, because we want to do it before any data is saved anywhere. This is a tricky issue though.