Refactoring: move GUID operations out of Joystick

- SDL_JoystickGUID -> SDL_GUID (though we retain a type alias)
- Operations for GUID <-> String ops are now in
  src/SDL_guid.c and include/SDL_guid.h
- The corresponding Joystick operations delegate to SDL_guid.c
- Added test/testguid.c

Makefile.in

@@ -80,6 +80,7 @@ HDRS = \
 	SDL_filesystem.h \
 	SDL_gamecontroller.h \
 	SDL_gesture.h \
+	SDL_guid.h \
 	SDL_haptic.h \
 	SDL_hidapi.h \
 	SDL_hints.h \