[vtkusers] Added QVTKRenderWindowInteractor.py

[Prabhu Ramachandran]

hi,

I managed to get a QVTKRenderWindowInteractor.py working based on the
David's QVTKRenderWidget.py.  I just committed it and its in the
vtk/qt/ directory.  This completes the interactor based widgets for
all the presently supported Python widgets.  It seems to work fine for
me but YMMV.  Note that just like QVTKRenderWidget it is supposed to
work only under *nix and not under Win32.

prabhu