Handle self-edges, duplicated nodes, and duplicated edges in graph viewer

[grtlr]

Right now we can't draw self-edges. Also, our model around how to handle duplicated nodes and duplicated edges is ambiguous.

@abey79 I think the following makes sense:

• Duplicated nodes is undefined behavior.
• Self-edges are ignored by the force-based layout algorithm but shown in the graph view, with a length depending on the distance parameter (potentially multiplied for multiple self edges).

[abey79]

Makes sense to me 👍🏻