[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/MecanumControllerCommand/cpp/RobotContainer.cpp

@@ -30,10 +30,9 @@ RobotContainer::RobotContainer() {
   // Set up default drive command
   m_drive.SetDefaultCommand(frc2::RunCommand(
       [this] {
-        m_drive.Drive(m_driverController.GetY(frc::GenericHID::kLeftHand),
-                      m_driverController.GetX(frc::GenericHID::kRightHand),
-                      m_driverController.GetX(frc::GenericHID::kLeftHand),
-                      false);
+        m_drive.Drive(m_driverController.GetLeftY(),
+                      m_driverController.GetRightX(),
+                      m_driverController.GetLeftX(), false);
       },
       {&m_drive}));
 }
@@ -42,7 +41,8 @@ void RobotContainer::ConfigureButtonBindings() {
   // Configure your button bindings here
 
   // While holding the shoulder button, drive at half speed
-  frc2::JoystickButton(&m_driverController, 6)
+  frc2::JoystickButton(&m_driverController,
+                       frc::XboxController::Button::kRightBumper)
       .WhenPressed(&m_driveHalfSpeed)
       .WhenReleased(&m_driveFullSpeed);
 }