Don't not hard code GraphicElement to Group

Let us choose how we handle FXG and change the parameters on GraphicElement.parentChanged() to and interface or just type it to DisplayObjectContainer. It clearly doesn't need to be a Group as the property is sets is typed as DisplayObjectContainer.

/**

• @private
• Storage for the parent property.
  */
  private var _parent:DisplayObjectContainer;

/**

• @inheritDoc
• @langversion 3.0
• @playerversion Flash 10
• @playerversion AIR 1.5
• @productversion Flex 4
  */
  public function get parent():DisplayObjectContainer { return _parent; }

/**

• @inheritDoc
• @langversion 3.0
• @playerversion Flash 10
• @playerversion AIR 1.5
• @productversion Flex 4
  */
  public function parentChanged(value:Group):void { _parent = value; }