Add support for building with VTK 9. QVTKOpenGLWidget is deprecated since VTK 9
and is now only a wrapper for QVTKOpenGLStereoWidget [1]. Thus, use the preferred
and versatile QVTKOpenGLNativeWidget [2] instead for VTK_MAJOR_VERSION >= 8.

 * radixwidgets/vtkchartwidget.hh: include appropriate
   QVTKOpenGL{,Native}Widget.h header and create typedef QVTKWidget_t based on
   VTK version
 * radixwidgets/examples/radixvtkchartwidget.cc: likewise.
 * radixwidgets/examples/radixvtkerrorbars.cc: use QVTKWidget_t
 * radixwidgets/vtkchartwidget.cc: likewise.

[1] https://vtk.org/doc/nightly/html/classQVTKOpenGLWidget.html
[2] https://vtk.org/doc/nightly/html/classQVTKOpenGLNativeWidget.html