Add keyboard and mouse support

[icculus]

We have art we can use for this, might as well add it to the API, even if it isn't meant to be device-specific.

[ShockSlayer]

Would love to see this. By default I'm loading my own placeholder key images which I overwrite once a controller is detected. If the controller is lost (or the keyboard starts being used) it would be nice to be able to redraw using keys instead; or rather, draw the keys directly in the first place.