RFR: 8313956: focusWithin on parents of a newly-added focused node is not updated

[Michael Strauß]

This PR fixes an issue with the way `focusWithin` bits are adjusted in the scene graph. Previously, the `focusWithin` counts of all parents of a removed node would be decreased if the removed node has a non-zero `focusWithin` count. This PR adds logic for the opposite scenario: the `focusWithin` count is increased on all parents of a newly-added node that is already focused (or contains other focused nodes).

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Commit messages:
 - Update focus-within bits on parents of newly-added focused node
 - Added failing test

Changes: https://git.openjdk.org/jfx/pull/1210/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1210&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313956
  Stats: 54 lines in 2 files changed: 47 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1210.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1210/head:pull/1210

PR: https://git.openjdk.org/jfx/pull/1210