RFR: 8350048: Enforce threading restrictions for show and hide methods in Window, Control, and Skin [v7]

[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 two additional commits since the last revision:

 - indent
 - indent

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1717/files
  - new: https://git.openjdk.org/jfx/pull/1717/files/2f2caa35..8663425c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1717&range=06
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1717&range=05-06

  Stats: 16 lines in 8 files changed: 0 ins; 0 del; 16 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