Make the automatic layout size-aware

[cb109]

Currently the width of a Node is dictated by its header text, which actually is an okayish estimate that graphviz can work with. The height however is set to 0.5 by default and does not take into account the actual height of the Node, which is dictated by its number of Knobs.

pydot most likely supports setting widthand height on a Node. Currently only Edges are created from which graphviz itself derives the Nodes. We should add them explicitly so we can modify them before handing to graphviz for layouting.