Move entityclass::settreadmillcolour() to entclass::settreadmillcolour()

This moves settreadmillcolour() onto the entity object, so I can invoke
it independent of an indice in obj.entities.

desktop_version/src/Ent.h

@@ -16,6 +16,8 @@ public:
 
     void setenemyroom(int rx, int ry);
 
+    void settreadmillcolour(int rx, int ry);
+
 public:
     //Fundamentals
     bool active, invis;