Support for keyboard keys on overlays.

This is pretty straightforward, keys can be added by setting the input string to one of the RETROK constants (converted to lowercase).

I included media/overlays/qwerty/qwerty.cfg as a rough example. The key labels in qwerty.png are blurry, inappropriate and clipped because I generated it using the geometry section of an XKB layout. I will clean up the code I used to make it and post it somewhere later.