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