SDL2 SDL_JOYDEVICEADDED and SDL_JOYDEVICEREMOVED undocumented features implementation

First implementation of SDL_JOYDEVICEADDED and SDL_JOYDEVICEREMOVED features. We maybe would not need to iterate over SDL_NumJoysticks anymore, as when SDL first inits the joystick subsystem, it emits
SDL_JOYDEVICEADDED events for every joystick currently on the
system.