[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/test/native/cpp/PS4ControllerTest.cpp

@@ -0,0 +1,37 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "frc/PS4Controller.h"  // NOLINT(build/include_order)
+
+#include "JoystickTestMacros.h"
+#include "frc/simulation/PS4ControllerSim.h"
+#include "gtest/gtest.h"
+
+using namespace frc;
+
+BUTTON_TEST(PS4Controller, SquareButton)
+BUTTON_TEST(PS4Controller, CrossButton)
+BUTTON_TEST(PS4Controller, CircleButton)
+BUTTON_TEST(PS4Controller, TriangleButton)
+
+BUTTON_TEST(PS4Controller, L1Button)
+BUTTON_TEST(PS4Controller, R1Button)
+BUTTON_TEST(PS4Controller, L2Button)
+BUTTON_TEST(PS4Controller, R2Button)
+
+BUTTON_TEST(PS4Controller, ShareButton)
+BUTTON_TEST(PS4Controller, OptionsButton)
+
+BUTTON_TEST(PS4Controller, L3Button)
+BUTTON_TEST(PS4Controller, R3Button)
+
+BUTTON_TEST(PS4Controller, PSButton)
+BUTTON_TEST(PS4Controller, Touchpad)
+
+AXIS_TEST(PS4Controller, LeftX)
+AXIS_TEST(PS4Controller, RightX)
+AXIS_TEST(PS4Controller, LeftY)
+AXIS_TEST(PS4Controller, RightY)
+AXIS_TEST(PS4Controller, L2Axis)
+AXIS_TEST(PS4Controller, R2Axis)