General improvements for user custom event registration * Switched event type to enum (int32) * Switched polling by mask to polling by type range * Added SDL_RegisterEvents() to allow dynamic user event registration * Spread events out to allow inserting new related events without breaking binary compatibility * Added padding to event structures so they're the same size regardless of 32-bit compiler structure packing settings * Split SDL_HasEvent() to SDL_HasEvent() for a single event and SDL_HasEvents() for a range of events * Added SDL_GetEventState() as a shortcut for SDL_EventState(X, SDL_QUERY) * Added SDL_FlushEvent() and SDL_FlushEvents() to clear events from the event queue