Fix stylesheet interpretation

[epost]

problem

Styling from the stylesheet will disappear for some nodes.

observations:

• node order should be preserved in the general case (eg for sequence diagrams and graphviz)

• a node can occur in grasp/graphviz either:

  • specified explicitly as a node, either
    • styled
    • unstyled
  • specified only as part of an edge

• in GraphViz and other output formats, order is significant, for example in that styling for later nodes/edges overrides that of earlier ones.

towards a fix

• a node that occurs only as part of an edge may still have styling specified in the stylesheet
  • in that case, for graphviz, we should synthetically insert the node as an explicit node, so we can attach styling to it