Overlay previous positions of vertices to show movement

[muellerbjoern]

Idea: At one point select vertices and save their positions, these current positions will be overlayed when the simulation goes on. The original vertices just move around as usual while 'ghost vertices' remain at the fixed positions so the positions can be compared. This enables the user to really see where exactly a vertex came from.
Maybe for small numbers of vertices some kind of trailing could even be implemented, making it possible to see the actual trajectories of vertices.