* moved the ASCII conversion hackery in SDLayer to a subfunction because this made things just messy.
* integrated the keyboard callback's functionality directly into inputState for consolidation purposes. This was yet another independent layer in the keyboard management.
* hook up D_PostEvent as the central place to dispatch keyboard input. This is now the only function that is getting called from the backend and a major prerequiside for swapping out the backend for GZDoom's.
Todo: Route mouse input through that, too.
