RFR: 8365615: Improve JMenuBar/RightLeftOrientation.java

[Alexey Ivanov]

Position the two frames with menu bars in left-to-right and right-to-left orientations below the frame that allows changing the look and feel.

Additionally, amend the instructions and refactor the code, show an error message to the tester if setting L&F fails.

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Commit messages:
 - 8365615: Improve JMenuBar/RightLeftOrientation.java

Changes: https://git.openjdk.org/jdk/pull/26803/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26803&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365615
  Stats: 56 lines in 1 file changed: 17 ins; 10 del; 29 mod
  Patch: https://git.openjdk.org/jdk/pull/26803.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26803/head:pull/26803

PR: https://git.openjdk.org/jdk/pull/26803