RFR: 8343336: Add persistentScrollBars preference [v6]

[Andy Goryachev]

On Wed, 27 Nov 2024 00:26:16 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> This PR adds another accessibility preference that is available on all supported desktop platforms:
>> 
>> 1. **Windows**: Settings -> Accessibility -> Visual Effects -> Always show scroll bars
>> 2. **macOS**: System Settings -> Appearance -> Show scroll bars
>> 3. **Ubuntu**: Settings -> Accessibility -> Seeing -> Always show scroll bars
>> 
>> When the OS setting is changed, all platforms report updates that can be observed in JavaFX.
>> The manual test application `test/manual/events/PlatformPreferencesChangedTest` can be used to test the feature.
>
> 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 six additional commits since the last revision:
> 
>  - Merge branch 'master' into feature/accessibility-hide-scrollbars
>  - check whether gtk-overlay-scrolling exists
>  - Merge branch 'master' into feature/accessibility-hide-scrollbars
>  - add platform keys to javadoc
>  - Change reducedScrollBars to persistentScrollBars
>  - Add reducedScrollBars preference

ship it!

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Marked as reviewed by angorya (Reviewer).

PR Review: https://git.openjdk.org/jfx/pull/1618#pullrequestreview-2473467428