Graph rendering: Prevent overlapping edge ends

[this-is-sofia]

At the moment, multiple edges can end at the same point of a node and therefore it can be hard to differentiate which end belongs to which edge. This can be a problem when edges are of different types (partially directed, bidirected, undirected) and might lead to confusion.

Possible solutions would be curved edges, space between the edge ends or at least including the info about the edge type in the widget that pops up when you click on the edge.