code: function( contentItem, index ) {
  		if ( index === undefined ) {
  			index = this.contentItems.length;
  		}
  
  		this.contentItems.splice( index, 0, contentItem );
  
  		if( this.config.content === undefined ) {
  			this.config.content = [];
  		}
  
  		this.config.content.splice( index, 0, contentItem.config );
  		contentItem.parent = this;
  	}
docs: Sets up the tree structure for the newly added child
  The responsibility for the actual DOM manipulations lies
  with the concrete item
  
  @param {lm.items.AbstractContentItem} contentItem
  @param {[Int]} index If omitted item will be appended