Aircraft inherits from Entity which inherits from SceneNode.  Through this
inheritance the access specifer was inadvertently change from private to
public.  This is the fix.
Chapter 6 - added a GUI framework which includes containers, labels, and
buttons.  The container logically bind components together (base class of
labels and buttons).  The container also tracks which component is
selected, so when a button is pressed we know what button to act on.