Remove mpGameScene member from KBlocksView

KBlocksView actually stored two pointers to the same Scene object. The
parent QGraphicsView stores a QGraphicsScene pointer, accessible via
scene() and setScene(). And KBlocksView also stored a SceneInterface
pointer as a private member function.

This could lead to confusing behavior. For example, calling setScene()
on the KBlocksView object only replaces one of the two pointers. I have
removed the private mpGameScene member. The scene is now always accessed
via the scene() interface.