RFR: 8350048: Enforce threading restrictions for show and hide methods in Window, Control, and Skin [v7]

[Michael Strauß]

On Fri, 7 Mar 2025 17:56:42 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

>> - enforced fx application thread
>> - clarify doc where an `IllegalStateException` can get thrown, such as hide() and show()
>> - clarified `ComboBoxBase::show`
>> - added a headful test `TestThreadingRestrictions`
>
> Andy Goryachev has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - indent
>  - indent

Marked as reviewed by mstrauss (Reviewer).

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PR Review: https://git.openjdk.org/jfx/pull/1717#pullrequestreview-2667965598