Workaround to fix the Command / Apple modifier key functionality on Darwin / macOS.

[Blady-Com]

-- macOS (at least 12.0) fires MOD2 and META key modifiers -- as we want only META we suppress MOD2 to the default mask Honestly, I wasn't able to determine where is the root cause: either macOS, GTK or GTKAda. Thus the workaround is applied at GNATStudio level.