feat(form): add $dirtyAfter state to form controls

[troch]

This is my attempt to add features to client side validation and inline form validation.

The aim of having a $dirtyAfter state per control is to track user's progress through the form and not display validation messages prematurely.

[troch]

Any feedback on this?

[Narretz]

@troch It looks like the new $touched / $untouched props solve a similar problem: adcc5a0
Do you agree, or did you try to achieve something else with dirtyAfter? The only real difference I see is that $touched / $untouched is not reset when calling $setPristine, and that it uses real events rather than reacting to $setViewValue

[troch]

It is slightly different: the idea of $dirtyAfter is not to give status information about the current form element, but to check if any form element after the current one is dirty.

In practice, this could be used to show inline validation messages on a form element which has been skipped by a user.

[Narretz]

I've decided that this will not be implemented. It adds a layer of complexity to the ngModel / form API that not many poeple will benefit from. I also doubt that showing messages for skipped form fields is a common enough use case. Either your form fields need to be filled out in a specific order - then you have to implement the restriction anyway - or they don't have to be,