RFR: 8350976: MenuBarSkin: exception initializing in a background thread

[Andy Goryachev]

Allows MenuBar to be created in a background thread by delaying MenuBarSkin::rebuildUI() call until after the MenuBar becomes a part of the scene graph.

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Commit messages:
 - spelling
 - use system menu
 - cleanup
 - possible fix
 - test

Changes: https://git.openjdk.org/jfx/pull/1727/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1727&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350976
  Stats: 67 lines in 2 files changed: 56 ins; 7 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1727.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1727/head:pull/1727

PR: https://git.openjdk.org/jfx/pull/1727