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/Entity.h

@@ -80,8 +80,6 @@ public:
 
     int crewcolour(int t);
 
-    void settreadmillcolour(int t, int rx, int ry);
-
     void createentity(float xp, float yp, int t, float vx = 0, float vy = 0,
                       int p1 = 0, int p2 = 0, int p3 = 320, int p4 = 240 );