Initial Apple TV / tvOS support.

The Apple TV remote is currently exposed as a joystick with its touch surface treated as two axes. Key presses are also generated when its buttons and touch surface are used.

A new hint has been added to help deal with deciding whether to background the app when the remote's menu button is pressed: SDL_HINT_APPLE_TV_CONTROLLER_UI_EVENTS.

include/SDL_hints.h

@@ -276,7 +276,25 @@ extern "C" {
  *    "LandscapeLeft", "LandscapeRight", "Portrait" "PortraitUpsideDown"
  */
 #define SDL_HINT_ORIENTATIONS "SDL_IOS_ORIENTATIONS"
-    
+
+/**
+ *  \brief  A variable controlling whether controllers used with the Apple TV
+ *  generate UI events.
+ *
+ * When UI events are generated by controller input, the app will be
+ * backgrounded when the Apple TV remote's menu button is pressed, and when the
+ * pause or B buttons on gamepads are pressed.
+ *
+ * More information about properly making use of controllers for the Apple TV
+ * can be found here:
+ * https://developer.apple.com/tvos/human-interface-guidelines/remote-and-interaction/
+ *
+ *  This variable can be set to the following values:
+ *    "0"       - Controller input does not generate UI events (the default).
+ *    "1"       - Controller input generates UI events.
+ */
+#define SDL_HINT_APPLE_TV_CONTROLLER_UI_EVENTS "SDL_APPLE_TV_CONTROLLER_UI_EVENTS"
+
 /**
  *  \brief  A variable controlling whether the Android / iOS built-in
  *  accelerometer should be listed as a joystick device, rather than listing