get_preferred_locales

[sanette]

Implements (issue #110)

https://wiki.libsdl.org/SDL2/SDL_GetPreferredLocales

Here is what it does on my machine:

# let l = Sdl.get_preferred_locales ();;
val l : Sdl.locale list Sdl.result =
  Ok
   [{Tsdl.Sdl.language = "fr"; country = Some "FR"};
    {Tsdl.Sdl.language = "fr"; country = None}]

I hope the code is OK, but I could not do much testing apart from the above.
I tried to take care of string memory management, but I did not check the Error case

[sanette]

to be complete one should also add the SDL_LOCALECHANGED event