RFR: 8350048: Enforce threading restrictions for show and hide methods in Window, Control, and Skin [v2]

[Andy Goryachev]

> - enforced fx application thread
> - added a headful test `TestThreadingRestrictions`
> 
> ## Note to the Reviewers
> 
> To avoid merge conflicts, the preferred order of integrations:
> 
> #1697 
> #1713 
> #1717

Andy Goryachev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - 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=01
  Stats: 465 lines in 11 files changed: 413 ins; 28 del; 24 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