[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

wpilibcExamples/src/main/cpp/examples/GearsBot/include/RobotContainer.h

@@ -4,7 +4,7 @@
 
 #pragma once
 
-#include <frc/Joystick.h>
+#include <frc/XboxController.h>
 #include <frc2/command/Command.h>
 
 #include "commands/Autonomous.h"
@@ -28,7 +28,7 @@ class RobotContainer {
 
  private:
   // The robot's subsystems and commands are defined here...
-  frc::Joystick m_joy{0};
+  frc::XboxController m_joy{0};
 
   Claw m_claw;
   Wrist m_wrist;