RFR: 8294589: MenuBarSkin: memory leak when changing skin [v2]

[Andy Goryachev]

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

Andy Goryachev has updated the pull request incrementally with one additional commit since the last revision:

  8294589: unnecessary null checks

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Changes:
  - all: https://git.openjdk.org/jfx/pull/906/files
  - new: https://git.openjdk.org/jfx/pull/906/files/1adbc9ad..7824baa8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=906&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=906&range=00-01

  Stats: 23 lines in 1 file changed: 1 ins; 7 del; 15 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