code: function forEachItem (callback) {
          this.items.each(function (item) {
              return ItemGroup.isItemGroup(item) ? item.forEachItem(callback) : callback(item, this);
          }, this);
      }
docs: Calls the callback for each item belonging to itself. If any ItemGroups are encountered,
  they will call the callback on their own Items.
  
  @private
  @param {Function} callback