Implement layout graphs

[dexterpratt]

Currently, the main, abstract graph is the layout graph. This is OK in some cases, but the protocol should be that the layout engine provides the graph to the visualizer and that graph might be different than the abstract graph. In which case, the layout engine must provide a mapping from the layout graph back to the abstract graph.

(the use case is where there are new nodes added for the purpose of layout, such as intermediate joints in edges)