Improve floating topic collision avoidance

Although overlapping floating topics would of course be undesirable, the collision avoidance algorithm is happy to swap the order of floating topics to resolve collisions. This is also undesirable where the sequence and positioning of topics is critical, e. g. in a process diagram. The results can be quite confusing. I would prefer it if the collision avoidance mechanism did its best to preserve the relative positions of floating topics by pushing others out of the way rather than jumping over them. I suspect this is easier said than done, though.