[jdk25u] RFR: 8361298: SwingUtilities/bug4967768.java fails where character P is not underline

[Trupti Patil]

- PR Changes
For Windows and GTK L&F, mnemonic is visible when ALT key is pressed.
Test instruction updated to reflect the desired behaviour.

- Testing
[JTREG Testing] (1 manual test ) - The results after running the test locally using jtreg are as expected.
runner starting test: javax/swing/SwingUtilities/bug4967768.java
runner finished test: javax/swing/SwingUtilities/bug4967768.java
Passed. Execution successful
Test results: passed: 1


- Conflicts
No conflicts. Clean backport.

- Are higher backports completed(25u,21u,17u,11u,8u etc) ?
Applicable to 25u via this MR and the others are being worked upon.

- Does it contain multiple changesets ?
No

-------------

Commit messages:
 - Backport 57553ca1dbc63e329116bc11764816a4c5ccb297

Changes: https://git.openjdk.org/jdk25u/pull/170/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=170&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361298
  Stats: 9 lines in 1 file changed: 6 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk25u/pull/170.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/170/head:pull/170

PR: https://git.openjdk.org/jdk25u/pull/170