Support :focus-visible CSS pseudoclass

[Michael Strauß]

> Michael: It does sound like this is a generally useful feature to do in
> the near future, so it makes sense to proceed with the review. I presume
> that the API as you have described it in the JBS description is what you
> are currently proposing? If so, I'll take a look at the API + docs in
> the next couple of days and we can take it from there.

Yes, the JBS issue has the latest version of my proposal for this feature:
https://bugs.openjdk.java.net/browse/JDK-8268225