Implement the WindowEventListener

[rgrieder]

Every time the size of a window changes, or someone even closes the window by the little square button with the cross, Orxonox has to notice that and act accordingly. The most important task right now is to reset the mouse clipping size (setWindowExtens in the InputHandler) or else our ship will be unsteerable if the window is being resized.
For references how this can be done look into the Ogre wiki or have a quick look at the ExampleFrameListener that already implements WindowEventListener.