New implementation of QVTKOpenGLWidget|QVTKOpenGLWindow class based on QOpenGLWindow
Fixing stereo support with Qt5.
139f787a introduced QVTKOpenGLWidget to couple a vtkRenderWindow with a QOpenGLWidget. However, both QVTKOpenGLWidget and its superclass QOpenGLWidget manage a single frame buffer.
QVTKOpenGLWindow derives the QOpenGLWindow class to benefit from an OpenGL context managing multiple frame buffers. We define a QVTKOpenGLWidget2 that provides a direct wrapper around QVTKOpenGLWindow, to use it as a QWidget.
The old QVTKOpenGLWidget is been moved into QVTKOpenGLSimpleWidget and should not be removed as it support being a native widget, wich the new implementation does not.