JavaFX Preview Features

[mstr2]

This PR contains a proposal for JavaFX Preview Features.

This PR is illustrative in nature. It will remain in Draft and will never be made rfr. It includes the following:

1. The JavaFX Preview Features JEP and implementation notes.
2. Two sample preview features: PreviewFeature.MY_SAMPLE_FEATURE_1 and PreviewFeature.MY_SAMPLE_FEATURE_2, along with the preview APIs javafx.application.MyPreviewFeature, Platform.getMyPreviewFeature1(), and Platform.getMyPreviewFeature2().

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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/1700/head:pull/1700
$ git checkout pull/1700

Update a local copy of the PR:
$ git checkout pull/1700
$ git pull https://git.openjdk.org/jfx.git pull/1700/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 1700

View PR using the GUI difftool:
$ git pr show -t 1700

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jfx/pull/1700.diff

[bridgekeeper]

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[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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