RFR: 8155030: The Menu Mnemonics are always displayed for GTK LAF [v7]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Remove public API and test update
 - Merge
 - condition update
 - Review comment fix
 - condition check update
 - Alt press property added for GTK L&F
 - Var moved to local scope
 - Mnemonic toggle fix

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18992/files
  - new: https://git.openjdk.org/jdk/pull/18992/files/47d4c18c..dc9465ac

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18992&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18992&range=05-06

  Stats: 265947 lines in 5365 files changed: 151720 ins; 84798 del; 29429 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