RFR: 8301302: Platform preferences API [v21]

[Michael Strauß]

> Please read [this document](https://gist.github.com/mstr2/9f46f92c98d3c86aa6a0b4224a9a6548) for an introduction to the Platform Preferences API, and how it interacts with the proposed style theme and stage appearance features.

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

  review changes

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1014/files
  - new: https://git.openjdk.org/jfx/pull/1014/files/ad74e397..38177a8d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1014&range=20
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1014&range=19-20

  Stats: 22 lines in 1 file changed: 17 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jfx/pull/1014.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1014/head:pull/1014

PR: https://git.openjdk.org/jfx/pull/1014