/**
* set collapsibility property for this field group.
*
* @param {boolean} collapsible if true then this field group is collapsible
* @param {boolean} [defaultcollapsed] if true and the field group is collapsible, collapse this field group by default
* @return {nlobjFieldGroup}
*
* @method
* @memberOf nlobjFieldGroup
*
* @since 2009.2
*/