Required library edition:
Required library file: dhtmlxmenu_ext.js

adds a new checkbox, sibling|child mode

• mode - (string) sibling|child
• nextToId - the item after which the checkbox will be added in the “sibling” mode or parent item's id in the “child” mode
• pos - item's position in the child mode (null for sibling)
• itemId - id of a new checkbox
• itemText - text of a new checkbox
• state - checked|unchecked
• disabled - enabled|disabled

    menu.addCheckbox(mode, nextToId, pos, itemId, itemText, state, disabled);