RFR: 8155030: The Menu Mnemonics are always displayed for GTK LAF [v15]

[Abhishek Kumar]

> In GTK LAF, the menu mnemonics are always displayed which is different from the native behavior. In native application **(tested with gedit for normal buttons and tested with libreoffice for menu**), 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 is green and link attached in JBS.

Abhishek Kumar has updated the pull request incrementally with one additional commit since the last revision:

  Javadoc style comment, unused import removed

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18992/files
  - new: https://git.openjdk.org/jdk/pull/18992/files/4aa6db65..81259a7b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18992&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18992&range=13-14

  Stats: 12 lines in 5 files changed: 2 ins; 8 del; 2 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