Fix mouse wheel access not working

Mouse wheel access should work now, the call to
window.addMouseWheelListener() was missing before.
GUI: Layout managing foundation

This commit adds the foundation for a GUI layout managing system closely
similar to the one AWT provides (so it isn't necessary to learn many new
techniques). Currently, two layouts are available, FlowLayout and
BorderLayout.
Coordinate-based positioning is still possible, and probably always will
be - this system is intended for more complex interfaces.