RFR: 8301302: Platform preferences API [v6]

[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 two additional commits since the last revision:

 - Revert Application changes
 - Addressed review comments

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1014/files
  - new: https://git.openjdk.org/jfx/pull/1014/files/40e2f286..4b9f75d4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1014&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1014&range=04-05

  Stats: 10 lines in 4 files changed: 1 ins; 3 del; 6 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