Interactive force-directed graph layout in Ingen

GraphViz has, unfortunately, broken reverse compatibility of their library API, which has been nagging as a maintenance headache for FlowCanvas and its successor Ganv for a while now. It would be nice to just get rid of that dependency altogether...

Meanwhile, I had to write some force-directed graph layout (FDGL) code for another project...

... You can probably see where this is going. In the spirit of it theoretically being a holiday (there are no holidays in practice in grad school), I spent some time implementing live FDGL in Ganv, and recorded a video of interactive layout in Ingen to show progress.

One nice thing about using live FDGL in a program like Ingen is that the user can customize the layout in an intuitive tactile way, since objects behave physical laws. This video demonstrates some manual rearranging of a patch, and a total reconfiguration by switching the signal flow direction. Manual tweaking of the layout like this was not possible with the previous GraphViz-based layout code.

It took quite a bit of tweaking to get the physics working well, so layouts looked good but the patch didn't explode if there are disconnected components. It's still not quite ideal, but usable.

This is implemented at the Ganv level and thus works in Ingen (as shown), Patchage, and Machina. I'm still not confident enough in this implementation to enable it by default and/or drop Graphviz quite yet, but the adventurous who follow svn, ./waf configure --debug --fdgl. If you're hit by the graphviz breakage (and I still haven't fixed the check...), add --no-graphviz to work around the problem.