A door is a mazeObject in a room. Each door has its selfId and a
belongId which is the id of the room which it belongs to.
A Door has a selfId and belongId to identify itself and the room it belongs to. The variable Side, Start, and End specifies which side it is in the room and the starting and end position at that side.
The method producingMessage() is to produce a MsgDoor message of this door.
Different Drawings of doors are added for different side of the rooms.