vtk : corrected a typo and added QVTKWidget2 wrapping

I corrected a typo in the vtk formula, it should be "qt" instead of "pyqt", as there is no more a "pyqt" option (everything gets build with the --with-qt option now).

I also added the line to build QVTKWidget2 wrappers for python (by setting VTK_USE_QVTK_QTOPENGL to ON)