Display edges in DotGraphModel somewhat correctly

Signed-off-by: Nicolai Hähnle <haehnle@or.uni-bonn.de>
Edge display in simple example

Signed-off-by: Nicolai Hähnle <haehnle@or.uni-bonn.de>
Add DotGraphDelegate

Signed-off-by: Nicolai Hähnle <haehnle@or.uni-bonn.de>