gambatte_sdl: reduce variable scope
gambatte_sdl: name constants
libgambatte/inputgetter: name enum
gambatte_sdl: encapsulate button mapping within inputoption
gambatte_sdl: use for_each to print option usage
gambatte_sdl: lower case constants/enums
gambatte_sdl: replace syncfunc with framewait object
gambatte_sdl: rename audiodata to audiosink
gambatte_sdl: enable failed field to be immutable
gambatte_sdl: define dtor to allow forward declaration