[Rev 02] 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.

Previous commits in this pull request have been removed, probably due to a force push. The incremental views will show differences compared to the previous content of the PR.

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Added commits:
 - 811035a6: Move import statement

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/86/files
  - new: https://git.openjdk.java.net/jfx/pull/86/files/94b2dba1..811035a6

Webrevs:
 - full: https://webrevs.openjdk.java.net/jfx/86/webrev.02
 - incr: https://webrevs.openjdk.java.net/jfx/86/webrev.01-02

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 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