Integration of Vegascene.

Change-Id: I3865ee2be442e81ab08006412d87e840cce659f6
vtkVegaSpecReader reads a spec file and generates the scene graph.

I provided the `vtkVegaSpecReader` class with new methods. Now the class can effectively read Vega spec data from a file or an input string and generates the related scene graph, which is possible to get in JSON format as a string.

  // Description:
  // In case a Vega spec file or string is available and generate the related
  // scene graph.
  virtual void Update();

  // Description:
  // Return the scene graph, related to the last rendering, in JSON format
  // as a string. If Update has never been invoked return a null string.
  std::string GetSceneString() const;

Change-Id: I8d24d5ef0ef1832deb76a94260c2eb26951c6161