RFR: 8294589: MenuBarSkin: memory leak when changing skin

[Andy Goryachev]

Fixed memory leak by using weak listeners and making sure to undo everything that has been done to MenuBar/Skin in dispose().

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Commit messages:
 - 8294589: cleanup
 - 8294589: executeOnceWhenPropertyIsNonNull
 - 8294589: review comments
 - 8294589: fixed leak test
 - 8294589: menu bar skin memory leak

Changes: https://git.openjdk.org/jfx/pull/906/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=906&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294589
  Stats: 255 lines in 5 files changed: 166 ins; 47 del; 42 mod
  Patch: https://git.openjdk.org/jfx/pull/906.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/906/head:pull/906

PR: https://git.openjdk.org/jfx/pull/906