title: Unable to clear a vtkRenderWindow
body: <p>I am using a QVTKWidget with a bunch of vtkImageMapper, vtkActor2D and two vtkRenderers.</p>
  
  <p>I load all these objects into the QVTKWidget with a DICOM image.</p>
  
  <p>There are two renderers because the user can open two images at the same time.</p>
  
  <p>However, he might want to close an image um order to replace it. The problem is i cant get the old image out of the window.</p>
  
  <p>The method Clear() from the render Windows apparently doesn't work. The same with RemoveAllProps().</p>
  
  <p>So, how can i clear the Windows?</p>