Add 'repeat' field to KeyDown and KeyUp events.

I noticed that SDL_KeybaordEvent (https://wiki.libsdl.org/SDL_KeyboardEvent) has a 'repeat' field, but it not being copied to KeyDown and KeyUp events. It would be great if I could access this value. This PR just adds a bool field to those events. One thing I'm not sure on though is whether having a repeated KeyUp event is actually a valid thing.