Switch SDL2_KeyboardUtil back to using keycodes

The ``SDL2_KeyboardUtil.ToXNA`` method should use keycodes rather than scancodes. XNA keys are layout-dependent, so using an AZERTY keyboard, when I press the A key (which is at the same geographical location as the Q key on a QWERTY keyboard), the key reported by XNA is ``Keys.A``, not ``Keys.Q``. FNA on the other hand, currently erroneously reports ``Keys.Q``

I went spelunking into the history and found that you transitioned from keycodes to scancodes a while back but I found no motivation for that change in the commit log (obviously you also rewrote the whole thing to use a map rather than a big fat switch): https://github.com/flibitijibibo/FNA/commit/5dbe6fc854fc8d2420ef327196ea8370942b3d3a