Adding Layer.update().

This method must be implemented by all subclasses of Layer.  It provides
the Layer an opportunity to update its underlying data structures before
being drawn.
Removing an unnecessary method from GridLayer.
Adding RootLayer.

Layers are now nested into each other, with the RootLayer containing all
Layers beneath.

KeyRunner's implementation of RootLayer contains the InfoBarLayer and
the GridLayer.  More layers to follow.