new Version 3.0.6

graphviz >= 2.30.0 has WITH_CGRAPH conditional:

struct Agraph_t gets defined different depending on version of grapviz is >= 2.30.0 or not.

predeclaration  of
   * struct Agraph_t
   * typedef struct Agraph_t graph_t
will lead to build-errors when building qt-graph
so let's include the proper define from graphviz

another minor change:
   * get rid of unused-var-error from examples/ManyWidgets.cc