Support :focus-visible CSS pseudoclass

[Michael Strauß]

I have added the following sentences to the JBS issue to clarify that
there will be no change for existing applications:

> The new "focusVisible" and "focusWithin" properties do not change the semantics of the existing "focused" property.
> Existing JavaFX themes will not be impacted by this change.

I will also note that the motivation for this proposal was primarily
to support modality-aware focus indication, which has historically
been more important for desktop applications than for web applications
(as evidenced by many desktop OS themes). So the cost-benefit tradeoff
is probably different than for the web.