RFR: 8355615: ConcurrentModificationException creating MenuBar on background thread

[Andy Goryachev]

Moving MenuBarSkin initialization code to install() which always happens in the FX Application Thread.

Also, ensure that the reverse process also happens in the FX application thread.

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Commit messages:
 - install

Changes: https://git.openjdk.org/jfx/pull/1795/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1795&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355615
  Stats: 23 lines in 1 file changed: 9 ins; 1 del; 13 mod
  Patch: https://git.openjdk.org/jfx/pull/1795.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1795/head:pull/1795

PR: https://git.openjdk.org/jfx/pull/1795