RFR: 8377706: [iOS] UI API called on a background thread: UIView layer

[Jose Pereda]

This PR adds a thread check to verify that native iOS UI code runs on the main thread, in a similar way as how `(void)setFrame:(CGRect)boundsRect` calls `_setBounds()`.

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Commit messages:
 - rename function
 - Move native iOS UI code to main thread

Changes: https://git.openjdk.org/jfx/pull/2073/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2073&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377706
  Stats: 10 lines in 1 file changed: 9 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/2073.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2073/head:pull/2073

PR: https://git.openjdk.org/jfx/pull/2073