Show the speedup between optimization steps in the transformation history of an SDFG

[phschaad]

The transformation history of an SDFG, together with the option of running SDFGs in profiling mode, give a clean overview of the optimization progress. However, one must manually keep track of the speedup between optimization steps.

Each item in the transformation history should thus have a graphical indicator to show the speedup measured over the last history entry before that.