Implement some sort of native form events

[joeholdcroft]

I often need a way to fire an event when a form is built (so listeners can add fields to it) and also when it is being processed (after passing validation) to do stuff with the data.

Can we support this natively within the form component? Symfony allows it like this but I dunno if that will work perfectly for us as the listeners are always added inside of buildForm() and we'd want to be able to do it from within bootstraps.

[joeholdcroft]

Closing this issue as we switched to Symfony form straight up and we can handle this with Symfony form now.

With regard to my concern about listeners being added with the form class, this won't be an issue so long as the form instances are in the service container. They can have listeners added publicly.