Display nested node graphs using graphviz' cluster syntax.
Handle internal relations of component nodes.
Invert edge direction to better reflect the meaning of a "dependency".
Show a legend in the graphviz output.
Color tweaks:
* Use explicit color values (hex format) instead of predefined schemes
for more control and larger palettes.
* Disabled: edge coloring (not very useful because hard to distinguish).
General support for subgraph nodes.