RFR: 8088594: NullPointerException on showing submenu of a contextmenu [v4]

[Karthik P K]

> When custom skin was loaded, the listeners added in `ContextMenuContent` class while loading the default skin were not removed. This was causing the NPE when outdated listeners were invoked.
> 
> Updated the code to dispose listeners in the `dispose` method of `ContextMenuSkin` so that when new skin is loaded, listeners added in the old skin are removed.
> 
> Added system test to validate the fix.

Karthik P K has updated the pull request incrementally with one additional commit since the last revision:

  Address code review

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1082/files
  - new: https://git.openjdk.org/jfx/pull/1082/files/cba6a855..f0d1e7f7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1082&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1082&range=02-03

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1082.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1082/head:pull/1082

PR: https://git.openjdk.org/jfx/pull/1082