RFR: 8268225: Support :focus-visible and :focus-within CSS pseudoclasses [v2]

[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 two additional commits since the last revision:

 - wording
 - Re-ordered methods

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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/7b3efd29..cf1daa2e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=475&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=475&range=00-01

  Stats: 65 lines in 2 files changed: 35 ins; 30 del; 0 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