RFR: 8301302: Platform preferences API [v4]

[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 with a new target base due to a merge or a rebase. The pull request now contains ten commits:

 - Added test
 - doc changes
 - Ensure that ColorProperty changes are atomic when observed
 - removed unused code
 - Move Appearance enum to javafx.application
 - doc changes
 - Removed platform-independent preference keys
 - documentation
 - Platform preferences implementation

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Changes: https://git.openjdk.org/jfx/pull/1014/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1014&range=03
  Stats: 3259 lines in 35 files changed: 3159 ins; 58 del; 42 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