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.

src/SDL.c

@@ -443,6 +443,8 @@ SDL_GetPlatform()
     return "Windows";
 #elif __WINRT__
     return "WinRT";
+#elif __TVOS__
+    return "tvOS";
 #elif __IPHONEOS__
     return "iOS";
 #elif __PSP__