[#States API] Basic Address field inherits wizard page's #states

Going to have a look at this issue.

I've gone ahead and pushed some code. It's definitely not complete, and I took the liberty of copy/pasting some code rather that introducing a helper method as described in #4 because for now, it might be useful to see where we need to introduce changes to get it to work properly.
It doesn't look as simple as letting stuff run on a composite element, because those elements aren't actually there at the point this code runs.
Once we've got the code working, I reckon we can clean it up to factor out common elements between the two code paths.

We're missing some tests for sure.

This code is currently working for my very simple use-case as described in the issue summary, so we should be able to construct a test that will then pass with this code.

I think there's a danger of all the edges cases though. So for example, at the moment I've skipped some of the copy/pasted code that would set an element to be visually hidden, because the elements aren't there to visually hide! But that might actually be okay, because if a wrapper is visually hidden, then that'll probably have the same effect.

Also, there's a todo about an after build that's getting added, maybe in the right way, I'm not sure yet!

I'm going to unassign myself, but might still come back around to look at getting a test sorted out at least.

Okay, this looks good, it seems we do have some tests for composite elements and states already: test_states_server_comp does seem to do a bit of this, and my new code is causing that to fail, which makes sense because I've left the bits that aren't working as TODOs, because yeah...they are hard/no idea how we're going to do that.

I still think we need an additional test that'll cover the case of a composite element on the second page of a form, where it's a required field, but won't currently be marked as such because it'll depend on the state of an element on a previous page. I'll work on getting such a test written up tomorrow, because yeah, the webform tests are seriously daunting if you've never worked on them before and it's taken me a few hours to get to grips with how it's all done.