RFR: 8268225: Support :focus-visible and :focus-within CSS pseudoclasses [v3]

[Michael Strauß]

> This PR adds the `Node.focusVisible` and `Node.focusWithin` properties, as well as the corresponding `:focus-visible` and `:focus-within` CSS pseudo-classes.

Michael Strauß has updated the pull request incrementally with one additional commit since the last revision:

  changes per discussion, added test

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Changes:
  - all: https://git.openjdk.java.net/jfx/pull/475/files
  - new: https://git.openjdk.java.net/jfx/pull/475/files/cf1daa2e..9d79df63

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=475&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=475&range=01-02

  Stats: 167 lines in 2 files changed: 89 ins; 61 del; 17 mod
  Patch: https://git.openjdk.java.net/jfx/pull/475.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/475/head:pull/475

PR: https://git.openjdk.java.net/jfx/pull/475