event_to_shortcut = function (event) {
        /**
         * Convert a jQuery Event object to a normalized shortcut string (shift-r)
         **/
        var shortcut = '';
        var key = inv_keycodes[event.which];
        if (event.altKey && key !== 'alt') {shortcut += 'alt-';}
        if (event.ctrlKey && key !== 'ctrl') {shortcut += 'ctrl-';}
        if (event.metaKey && key !== 'meta') {shortcut += 'meta-';}
        if (event.shiftKey && key !== 'shift') {shortcut += 'shift-';}
        shortcut += key;
        return shortcut;
    }
Return `true` if the event only contains modifiers keys.
false otherwise