RFR: 8349373: Support JavaFX preview features [v3]

[Michael Strauß]

> This PR contains a definition of preview features for JavaFX, as well as a helper class to verify that an application has opted into preview features.

Michael Strauß has updated the pull request incrementally with four additional commits since the last revision:

 - javadoc
 - remove supporting documentation
 - warning can't be suppressed
 - initialize PreviewFeature early

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1359/files
  - new: https://git.openjdk.org/jfx/pull/1359/files/2fa3560d..0a2a48b4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1359&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1359&range=01-02

  Stats: 65 lines in 4 files changed: 10 ins; 50 del; 5 mod
  Patch: https://git.openjdk.org/jfx/pull/1359.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1359/head:pull/1359

PR: https://git.openjdk.org/jfx/pull/1359