[DX] Add CSS classes to form elements that are hidden/shown via #states.

I was trying to figure out a way to solve #1968, but I see no way to target all these elements effectively in a single CSS rule. It would be easy if the #states elements had a generic .has-states class and perhaps also a second class with the specific state per element case. So a .states-[state] class where [state] would be one of: - enabled - disabled - required - optional - visible - invisible - checked - unchecked - expanded - collapsed - relevant - irrelevant - valid - invalid - touched - untouched - readwrite - readonly

Perhaps also a .states-lvl-x class if possible (where x is the numeric level of how many parent elements the element in question has).

I would file a PR, but this touches the Field API and I am not even remotely ready for than yet, so I rely on somebody else to tackle this. Once implemented, I think it will be easy(ier) for me to sort #1968