RFR: 8091673: Public focus traversal API for use in custom controls

[Andy Goryachev]

Public focus traversal API for use in custom controls.

https://github.com/andy-goryachev-oracle/Test/blob/main/doc/FocusTraversal/FocusTraversal-v3.md

This is a lightweight change that only adds the public API for focus traversal, containing neither the public API for the traversal policy (#1555) nor with the changes for the traversal policy hidden (#1604).

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Commit messages:
 - request focus traversal

Changes: https://git.openjdk.org/jfx/pull/1628/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1628&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8091673
  Stats: 85 lines in 2 files changed: 85 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1628.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1628/head:pull/1628

PR: https://git.openjdk.org/jfx/pull/1628