[Rev 01] 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.

The pull request has been updated with 1 additional commit.

-------------

Added commits:
 - 94b2dba1: Move import statement

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/86/files
  - new: https://git.openjdk.java.net/jfx/pull/86/files/7910ab27..94b2dba1

Webrevs:
 - full: https://webrevs.openjdk.java.net/jfx/86/webrev.01
 - incr: https://webrevs.openjdk.java.net/jfx/86/webrev.00-01

  Stats: 2 lines in 1 file changed: 1 ins; 1 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