RFR: 8236839: System menubar removed when other menubars are created or modified

[Johan Kaving]

This pull request fixes the sceneProperty listener in `MenuBarSkin` so that we leave the
current system menubar alone when other menubars are changed.

It also adds a test case that reproduces the problem before the fix.

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Commits:
 - 7910ab27: Don't always remove system menubar

Changes: https://git.openjdk.java.net/jfx/pull/86/files
 Webrev: https://webrevs.openjdk.java.net/jfx/86/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8236839
  Stats: 33 lines in 2 files changed: 32 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/86.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/86/head:pull/86

PR: https://git.openjdk.java.net/jfx/pull/86