RFR: 8350048: Enforce threading restrictions for show and hide methods in Window, Control, and Skin [v6]

[Andy Goryachev]

> - 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 with a new target base due to a merge or a rebase. The pull request now contains nine commits:

 - indent
 - Merge remote-tracking branch 'origin/master' into 8350048.enforce
 - Merge remote-tracking branch 'origin/master' into 8350048.enforce
 - review comments
 - review comments
 - Merge remote-tracking branch 'origin/master' into 8350048.enforce
 - fixed node init test
 - all tests
 - initial test

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Changes: https://git.openjdk.org/jfx/pull/1717/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1717&range=05
  Stats: 480 lines in 13 files changed: 425 ins; 28 del; 27 mod
  Patch: https://git.openjdk.org/jfx/pull/1717.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1717/head:pull/1717

PR: https://git.openjdk.org/jfx/pull/1717