RFR: 8354797: Parent.needsLayoutProperty() should return read-only getter

[Michael Strauß]

`Parent.needsLayout` is implemented with a `ReadOnlyBooleanWrapper`. The property getter returns the wrapper itself, but what it should be doing is return the read-only getter instead.

A single reviewer should be sufficient.

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Commit messages:
 - fix
 - failing test

Changes: https://git.openjdk.org/jfx/pull/1780/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1780&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354797
  Stats: 10 lines in 2 files changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1780.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1780/head:pull/1780

PR: https://git.openjdk.org/jfx/pull/1780