move highlighed edges to the front

[mateuszzz88]

I have a graph that is very crowded in some places. Highlighting node and its edges red (with search box) helps a lot. However It often happens that highlighted edge is below many not-highlighted and is hidden anyway.
I propose enhancement that red should be on top of black. Possibly it could also be slightly wider, but that is secondary.

Motivating example:
binfo_... is connected to CLUSTER_0, but it is very hard to see without magnifying glass

[jrfonseca]

Makes sense, it would indeed be better.

I'm afraid I don't have time to implement it. But if anybody wants to give a shot at a PR, I'd happily merge it.

I suppose that it would require to change xdot.py to do two passes over the graph: one for the stuff, another for the highlights. Currently we do only one pass IIRC.