RFR: 8155030: The Menu Mnemonics are always displayed for GTK LAF

[Abhishek Kumar]

In GTK LAF, the menu mnemonics are always displayed which is different from the native behavior. In native application **(tested with gedit**), the menu mnemonics toggle on press of `ALT` key. Menu mnemonics are hidden initially and then toggles between show/hide on `ALT` press. 
Proposed fix is to handle the `ALT` key press for GTK LAF and mimic the native behavior. Fix is similar to the `ALT` key processing in  Windows LAF. Automated test case is added to verify the fix and tested in Ubuntu and Oracle linux.

CI testing was green and link attached in JBS.

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Commit messages:
 - Mnemonic toggle fix

Changes: https://git.openjdk.org/jdk/pull/18992/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18992&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8155030
  Stats: 241 lines in 4 files changed: 237 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/18992.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18992/head:pull/18992

PR: https://git.openjdk.org/jdk/pull/18992