* 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.
This removes all unused parts of the implementation and moves the rest to the InputState class for easier replacement later. All MACT is doing now here is to call the UpdateStatus function, the internal workings are no longer relevant.
Having this grouped in a single object will make refactoring a lot easier.
Access functions have been provided to avoid changing client code for the reorganization.
