RFR: 8088594: NullPointerException on showing submenu of a contextmenu

Thu Apr 6 13:08:12 UTC 2023

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 This was causing the NPE when outdated listeners were invoked.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.

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Commit messages:
 - Add copyright text to fxml
 - Fix NPE in submenu of  context menu

Changes: https://git.openjdk.org/jfx/pull/1082/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1082&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8088594
  Stats: 342 lines in 6 files changed: 324 ins; 17 del; 1 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