Display different versions of the same query plan

[vogelsgesang]

Provide a way to select between multiple versions of the same query plan (e.g., different optmization steps) and use those for Hyper query plans which include multiple optimization steps

[rickcole]

User interface controls and/or keyboard/mouse to manage multiple graphs on a page would be great to have. Multiple graph support would also be useful for graph comparisons. Currently, one can only display different versions as one graph, e.g., in hyper/steps2.plan.json. To complete the list of related things, we have a rudimentary example of side-by-side comparison in compare-layout.html.