RFR: 8333589: [macos] Menu item actions do not work with Desktop.setDefaultMenuBar()

[Alisen Chung]

Add a check and storage for a custom DefaultMenuBar in Desktop.java and adds key event handling for a custom DefaultMenuBar in KeyboardManager.java.
Also adds a manual test for checking that key event handling is correct for DefaultMenuBars.

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Commit messages:
 - add test, fix defaultMenuBar menu item actions

Changes: https://git.openjdk.org/jdk/pull/19714/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19714&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333589
  Stats: 152 lines in 3 files changed: 152 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19714.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19714/head:pull/19714

PR: https://git.openjdk.org/jdk/pull/19714