RFR: 8368972: Create implementation of menu bar accessibility component [v3]

[Alexander Zuev]

> Create the accessibility peer for menu bar component also fix the problem when with the voiceover is on the <Ctrl>-F10 is pressed the name of the selected menu is not announced.

Alexander Zuev has updated the pull request incrementally with one additional commit since the last revision:

  Caching children list;
  Use nil instead of NULL;

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1927/files
  - new: https://git.openjdk.org/jfx/pull/1927/files/fe7d12bc..ef936ade

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1927&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1927&range=01-02

  Stats: 7 lines in 1 file changed: 1 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1927.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1927/head:pull/1927

PR: https://git.openjdk.org/jfx/pull/1927