RFR: 4795384: GTK+ JFileChooser buttons should have mnemonics

[Abhishek Kumar]

JFileChooser don't have the mnemonics for open and save button in GTK L&F. Fix is to add mnemonics for open and save button.
Fix can be verified by Swingset2 demo application. Mnemonics in GTK L&F are visible once alt key is pressed.
After running the SwingSet2 application, change the `L&F to GTK` and select` JFileChooser demo` from toolbar. Click on `Select Plain JFileChooser` button and then press alt key to verify the mnemonics.

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Commit messages:
 - add mnemonics for open and save button

Changes: https://git.openjdk.org/jdk/pull/20612/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20612&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-4795384
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20612.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20612/head:pull/20612

PR: https://git.openjdk.org/jdk/pull/20612