[wpilib] Add PS4Controller, remove Hand from GenericHID/XboxController (#3345)

- GenericHID is now concrete, and has only getRawAxis/Button(int) functionality
- getXxx() has been moved into Joystick as that's the only place where it makes sense
- Hand (and therefore getXxx(Hand)) has been removed, replaced by specific getLeft/RightXxx() methods in XboxController and the new PS4Controller class
- C++ ::Button:: and ::Axis:: enums have been converted to identically-namespaced static constexpr ints

wpilibc/src/main/native/cpp/Joystick.cpp

@@ -61,11 +61,11 @@ int Joystick::GetThrottleChannel() const {
   return m_axes[Axis::kThrottle];
 }
 
-double Joystick::GetX(JoystickHand hand) const {
+double Joystick::GetX() const {
   return GetRawAxis(m_axes[Axis::kX]);
 }
 
-double Joystick::GetY(JoystickHand hand) const {
+double Joystick::GetY() const {
   return GetRawAxis(m_axes[Axis::kY]);
 }