Allows you to loop through priority items in priority list.
Same function signature as Array.prorotype.forEach.
@param callback {Function} - Same signature as SjlMap.prorotype.forEach; I.e., {Function<value, key, obj>}.
@param context {undefined|*}
@method sjl.stdlib.PriorityList#forEach
@overrides sjl.stdlib.SjlMap#forEach
@returns {sjl.stdlib.PriorityList}
function (callback, context) {
            SjlMap.prototype.forEach.call(this.sort(), function (value, key, map) {
                callback.call(context, this.wrapItems ? value.value : value, key, map);
            }, this);
            return this;
        }