RFR: 8350048: Enforce threading restrictions for show and hide methods in Window, Control, and Skin [v4]

[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 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/7d8ef4e2..b6afe882

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1717&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1717&range=02-03

  Stats: 14 lines in 2 files changed: 12 ins; 0 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