Added SDL_ShowSimpleMessageBox function.
Added SDL_ShowSimpleMessageBox function.

There are two version of the function, one as a global function taking a *Window parameter (which can be nil) and the other as a method on *Window.

jangler/go-sdl2,veandco/go-sdl2,gcatlin/go-sdl2,Abextm/go-sdl2,Abextm/go-sdl2,krux02/go-sdl2,veandco/go-sdl2,phicode/go-sdl2,Abextm/go-sdl2,jangler/go-sdl2,krux02/go-sdl2,jangler/go-sdl2,phicode/go-sdl2,gcatlin/go-sdl2,gcatlin/go-sdl2,phicode/go-sdl2,krux02/go-sdl2