RFR: 8370498: Improve how Node detects whether a layout property change requires a new layout pass

[John Hendrikx]

This new check is much more accurate to detect whether a parent is currently laying out its children. The previous code almost never worked, resulting in additional unnecessary layouts.

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Commit messages:
 - Change how Node detects whether a relayout is required

Changes: https://git.openjdk.org/jfx/pull/1945/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1945&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370498
  Stats: 39 lines in 2 files changed: 21 ins; 12 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1945.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1945/head:pull/1945

PR: https://git.openjdk.org/jfx/pull/1945