RFR: 8350048: Enforce threading restrictions for show and hide methods in Window, Control, and Skin [v3]

[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 incrementally with one additional commit since the last revision:

  review comments

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1717/files
  - new: https://git.openjdk.org/jfx/pull/1717/files/db921097..7d8ef4e2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1717&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1717&range=01-02

  Stats: 13 lines in 4 files changed: 5 ins; 6 del; 2 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