RFR: 8319779: SystemMenu: memory leak due to listener never being removed [v6]

[Johan Vos]

> A listener was added but never removed.
> This patch removes the listener when the menu it links to is cleared. Fix for https://bugs.openjdk.org/browse/JDK-8319779

Johan Vos has updated the pull request incrementally with two additional commits since the last revision:

 - Cleanup test
 - Add shim class so that we can access the references to com.sun.glass.ui.Menu instances.
   Add a test to make sure those references are gone.

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1283/files
  - new: https://git.openjdk.org/jfx/pull/1283/files/9b1133cd..d5a78749

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1283&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1283&range=04-05

  Stats: 493 lines in 5 files changed: 317 ins; 175 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1283.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1283/head:pull/1283

PR: https://git.openjdk.org/jfx/pull/1283