message: Add foward declare for XmlElement
diff: #include "../Map/TileMap.h"
  +namespace NAS2D {
  + namespace Xml {
  + class XmlElement;
  + }
  +}
  +
  class Warehouse; /**< Forward declaration for getAvailableWarehouse() function. */
  class RobotCommand; /**< Forward declaration for getAvailableRobotCommand() function. */
  class RobotPool;
  