Preview features

[mstr2]

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.

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jfx.git pull/1359/head:pull/1359
$ git checkout pull/1359

Update a local copy of the PR:
$ git checkout pull/1359
$ git pull https://git.openjdk.org/jfx.git pull/1359/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 1359

View PR using the GUI difftool:
$ git pr show -t 1359

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jfx/pull/1359.diff

[bridgekeeper]

👋 Welcome back mstrauss! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[bridgekeeper]

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