RFR: 8343336: Add reducedScrollBars preference

[Michael Strauß]

This PR adds another accessibility preference that is available on all supported desktop platforms:

1. **Windows**: Settings -> Accessibility -> Visual Effects -> Always show scroll bars
2. **macOS**: System Settings -> Appearance -> Show scroll bars
3. **Ubuntu**: Settings -> Accessibility -> Seeing -> Always show scroll bars

When the OS setting is changed, all platforms report updates that can be observed in JavaFX.
The manual test application `test/manual/events/PlatformPreferencesChangedTest` can be used to test the feature.

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Commit messages:
 - Add reducedScrollBars preference

Changes: https://git.openjdk.org/jfx/pull/1618/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1618&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343336
  Stats: 203 lines in 18 files changed: 148 ins; 16 del; 39 mod
  Patch: https://git.openjdk.org/jfx/pull/1618.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1618/head:pull/1618

PR: https://git.openjdk.org/jfx/pull/1618