RFR: 8301302: Platform preferences API [v16]

[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 incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 25 additional commits since the last revision:

 - Merge branch 'master' into feature/platform-preferences
 - Handle key removals
 - Remove javadocs
 - Add signal handler for gtk-theme-name
 - change test class name
 - changes per review
 - Fire only a single invalidation event
 - Merge branch 'master' into feature/platform-preferences
 - Format arrays in test application
 - Suppress empty change message in test application
 - ... and 15 more: https://git.openjdk.org/jfx/compare/56ac1559...a893fa4c

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1014/files
  - new: https://git.openjdk.org/jfx/pull/1014/files/cb817623..a893fa4c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1014&range=15
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1014&range=14-15

  Stats: 10981 lines in 186 files changed: 8720 ins; 1349 del; 912 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