RFR: 8343336: Add persistentScrollBars preference [v3]

[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.

Michael Strauß has updated the pull request incrementally with one additional commit since the last revision:

  add platform keys to javadoc

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1618/files
  - new: https://git.openjdk.org/jfx/pull/1618/files/bd6ecbce..00de674f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1618&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1618&range=01-02

  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 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