RFR: 8350976: MenuBarSkin: exception initializing in a background thread [v2]

[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.

Andy Goryachev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge remote-tracking branch 'origin/master' into 8350976.menubarskin.thread.safety
 - spelling
 - use system menu
 - cleanup
 - possible fix
 - test

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1727/files
  - new: https://git.openjdk.org/jfx/pull/1727/files/97aafcb9..69a52e64

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1727&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1727&range=00-01

  Stats: 1488 lines in 19 files changed: 325 ins; 1034 del; 129 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