PS/2: Translate PS/2 codes to MC1-internal (more sane) scan codes

[mbitsnbites]

The PS/2 key codes do not really make any sense from a software perspective (e.g. alphanumeric characters are not encoded continuously in the key code space). Also, if we want to support other input devices (e.g. USB), we need a common, virtual encoding anyway.

For instance, consider using the GLFW key code encoding.