Fixes reparenting bug of QVTKWidget

As a follow-up of !2288 (closed), I have decided to still push the small changes to QVTKWidget and ask for merge.

I have noticed that Utkarsh has implemented the QVTKOpenGLWidget, that is great (even though it does not compile so far on my system), but it only works with OpenGL2. Since my application will be used a lot via Windows Remote Desktop (RDP), which only supports OpenGL1.1, I still need a working OpenGL1-based application.