<AWT Dev> RFR: 8208747: [a11y] [macos] In Optionpane Demo, inside ComponentDialog Example, unable to navigate to all items, with VO on

[Alexander Zuev]

Added accessibilityIndex function that correctly returns the index of a
child in parent container

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Commit messages:
 - 8208747: [a11y] [macos] In Optionpane Demo, inside ComponentDialog Example, unable to navigate to all items, with VO on

Changes: https://git.openjdk.java.net/jdk/pull/4084/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4084&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8208747
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4084.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4084/head:pull/4084

PR: https://git.openjdk.java.net/jdk/pull/4084