More progress on structures

COVER.md
src/SDL_KeyboardEvent.3
src/SDL_Keysym.3
src/SDL_MessageBoxButtonData.3
src/SDL_MessageBoxColor.3
src/SDL_MessageBoxColorScheme.3
src/SDL_MessageBoxData.3
src/SDL_MouseButtonEvent.3
src/SDL_MouseMotionEvent.3
src/SDL_MouseWheelEvent.3
src/SDL_MultiGestureEvent.3
src/SDL_Palette.3