RFR: 8294589: MenuBarSkin: memory leak when changing skin [v7]

[Andy Goryachev]

> Fixed memory leak by using weak listeners and making sure to undo everything that has been done to MenuBar/Skin in dispose().
> 
> This PR depends on a new internal class ListenerHelper, a replacement for LambdaMultiplePropertyChangeListenerHandler.
> See https://github.com/openjdk/jfx/pull/908

Andy Goryachev has updated the pull request incrementally with six additional commits since the last revision:

 - 8294589: removed weak listeners
 - Merge branch '8294809.listener.helper' into 8294589.menubarskin.leak
 - 8294809: removed weak listeners support
 - 8294809: use weak reference correctly this time
 - 8294809: tests
 - 8294809: remove

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Changes:
  - all: https://git.openjdk.org/jfx/pull/906/files
  - new: https://git.openjdk.org/jfx/pull/906/files/21fc352e..2ea27fc9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=906&range=06
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=906&range=05-06

  Stats: 765 lines in 3 files changed: 422 ins; 337 del; 6 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