[Input] Renamed Joy(x).button(y) to joy(x).button(y) for consistency
[Input] Renamed Joystick to JoystickButton
[Input] Added JoystickAxis and new corresponding action
[Input] JoystickButton, JoystickAxis: Renamed id variable to joystickId

This is more expressive and consistent with SFML's naming in the event structures
[Input] Joystick.hpp: Moved implementation to .cpp file