Callback was not removed when object was destroyed.
QVTKWidget added a callback to the RenderEvent but never removed it. Consequently, if the QVTKWidget was destroyed before the vtkRenderWindow, got a segfault due to dangling pointer. Fixed that. Also cleaned up the callback code to use the newer/safer/cleaner API to add callbacks for class methods instead of static functions. Change-Id: I2980f17aea8710802b3f51592479ee269eebdfad