Change visibility of sdl2::event::Event::from_ll to public

[jpteb]

As mentioned in #1023 I have a use case to convert a raw *mut sdl2_sys::SDL_Event into a sdl2::event::Event. Changing the visibility from sdl2::event::Event::from_ll to public would simplify this process greatly.

Other from_ll functions are already public and there doesn't seem to be a problem with changing others to public as well. It might even be worth thinking about setting all from_ll/to_ll functions to public.