RFR: 8319779: SystemMenu: memory leak due to listener never being removed [v21]

[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 one additional commit since the last revision:

  Allow scheduled runnables to be executed on the FX App Thread before checking for exceptions.

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1283/files
  - new: https://git.openjdk.org/jfx/pull/1283/files/8e30b998..39bf43da

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1283&range=20
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1283&range=19-20

  Stats: 10 lines in 1 file changed: 3 ins; 3 del; 4 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