Make use of `CallKeyCallback` for keyboard presses

Call the appropriate callback function when a given keyboard key is
pressed, held, or released.

There is little point listening to key states and allowing callbacks to
be set if the two are never linked during execution.

This step only makes use of callbacks for _keyboard_ events. While mouse
buttons (left, right, and middle buttons opposed to mouse movement and
wheel scrolling) behave in a similar, if not identical, way their code
is handled in `HandleMouseInput`.