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

[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

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Commit messages:
 - fixed node init test
 - all tests
 - initial test

Changes: https://git.openjdk.org/jfx/pull/1717/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1717&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350048
  Stats: 472 lines in 11 files changed: 429 ins; 14 del; 29 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