Add SDL_events.h definitions

Phew!

This is the first revision of the events stuff. It compiles, but I haven't tried running anything against it yet. It's also (mostly) documented, which is nice. There are a couple of changes elsewhere in the file to get things working properly: hopefully nothing too distressing.

I've got most of SDL_events.h here. There are a few events missing (Text events, Touch events and Gesture events) and a couple of functions (mostly relating to Event Filters, and I'm deathly afraid of trying to get function pointers working).

The SDL_Keysym struct also needs implementing, but that can come with the keyboard stuff, I guess.

-- David