RFR: 8349373: Support JavaFX preview features [v2]

[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 one additional commit since the last revision:

  move enum field to top

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1359/files
  - new: https://git.openjdk.org/jfx/pull/1359/files/925875b8..2fa3560d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1359&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1359&range=00-01

  Stats: 4 lines in 1 file changed: 2 ins; 2 del; 0 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