QVTK cursor change

vtkAbstractWidget and all its derivatives request the render window when the cursor shape needs to be changed. The platform-specific render window is then responsible for changing the cursor shape. With the new QVTKOpenGLWidget, QVTKOpenGLWindow and QVTKOpenGLNativeWidget classes, the render window is a vtkGenericOpenGLRenderWindow which is platform agnostic. The request is lost with the window.

This change ensures that the generic render window invokes a CursorChangedEvent each time a VTK class requests a cursor change. The QVTK classes then, via callback, request the cursor change via Qt.