the actual fix would be to make the argument of "agsafeset" take a "const char*", but this is in libgraphviz. cannot fix the whole world

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>

- First version of CMake support. - Also encapsulated all graphviz stuff into private objects, so that all public headers are free from graphviz headers - Defined QDECL_EXPORT macro for all public classes, to avoid auto import when using the library