get-default-shortcut-prefix in Miscellaneous
Returns an immutable list specifying the default prefix for menu
shortcuts. See also
get-shortcut-prefix in selectable-menu-item<%>.
Under Windows, the default is '(ctl). Under Mac OS X, the
default is '(cmd). Under X, the default is normally
'(ctl), but the default can be changed through the
'|MrEd:defaultMenuPrefix| preference low-level preference (see
Preferences).