RFR: JDK-6690019: JOptionPane obscured behind an always on top JFrame hangs UI

[Prasanta Sadhukhan]

A JOptionPane message dialog owned by a JFrame which is not always on top can be obscured behind an `"always on top" JFrame`. Since the option pane is modal, one can't move the "always on top frame" to reveal it, so the application effectively hangs as we cannot dismiss the option pane dialog
Fix is to make sure the option pane dialog is always on top....

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Commit messages:
 - JDK-6690019: JOptionPane obscured behind an always on top JFrame hangs UI

Changes: https://git.openjdk.org/jdk/pull/16060/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16060&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6690019
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16060.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16060/head:pull/16060

PR: https://git.openjdk.org/jdk/pull/16060