RFR: 8301302: Platform preferences API [v14]

[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:

 - Remove javadocs
 - Add signal handler for gtk-theme-name

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1014/files
  - new: https://git.openjdk.org/jfx/pull/1014/files/1eaea48a..aae55f99

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1014&range=13
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1014&range=12-13

  Stats: 58 lines in 4 files changed: 20 ins; 24 del; 14 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