RFR: 8343956: Focus delegation API [v2]

[Michael Strauß]

> Implementation of [focus delegation](https://gist.github.com/mstr2/44d94f0bd5b5c030e26a47103063aa29).

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

  Handling of several edge cases

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1632/files
  - new: https://git.openjdk.org/jfx/pull/1632/files/a9e6b8f8..6e00f8e5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1632&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1632&range=00-01

  Stats: 556 lines in 3 files changed: 463 ins; 40 del; 53 mod
  Patch: https://git.openjdk.org/jfx/pull/1632.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1632/head:pull/1632

PR: https://git.openjdk.org/jfx/pull/1632