Add GLUT key-press handling

Prior to this commit, the cursor was shown on screen, but there
was no capability to respond to key-presses made by the user.

This commit adds the key-press* group of functions, which act as
handlers for key-presses reported by the GLUT module.  Currently,
these handlers are only used for moving the cursor b/w squares
on the chess board.

Next steps along this line of implementation include adding key-selection
support, as a way to allow users to select/move pieces.