Add method to save GVGraph as DOT file
Add accessor for graph root id.
Add 'DocumentationMode' to GVGraph

For the documentation feature we need some special properties on
the graph, so we use a mode variable as switch for that.
Integrate the StateMachineDumper class

The StateMachineDumper class (better name needed!) generates a list of DOT
files, each describing one state (and all its children) and all outgoing transitions.