add html5 gamepad keycode mapping
implement some gamepad APIs
add JoyStickadded event handler
check eventType before adding joystick
set joystick added callback handler
check for value, not a constant
add a gamepad disconnected callback
set gamepad disconnected callback handler
add a JoystickByIndex helper function
implement SDL_SYS_JoystickNameForDeviceIndex
rename id to name
rename device_index to index
add device_instance to joystick
implement SDL_SYS_JoystickOpen
do some cleanup in case of failure
implement SDL_SYS_JoystickQuit
implement SDL_SYS_JoystickAttached
implement SDL_SYS_JoystickClose
implement JoystickGet*GUID
fix typo