RFR: 8155030: The Menu Mnemonics are always displayed for GTK LAF [v10]

[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:

  Common AltProcessor class for GTk and Aqua, helper method in MnemonicHandler class, review comments update and test extended for MacOS Aqua L&F

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18992/files
  - new: https://git.openjdk.org/jdk/pull/18992/files/e1aae36e..d8b10130

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18992&range=09
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18992&range=08-09

  Stats: 256 lines in 15 files changed: 109 ins; 77 del; 70 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