Xbox GDKX support (#5869)

* Xbox GDK support (14 squashed commits)

* Added basic keyboard testing

* Update readme

* Code review fixes

* Fixed issue where controller add/removal wasn't working (since the device notification events don't work on Xbox, have to use the joystick thread to poll XInput)

src/stdlib/SDL_malloc.c

@@ -487,7 +487,9 @@ DEFAULT_MMAP_THRESHOLD       default: 256K
 #endif /* WIN32 */
 
 #ifdef WIN32
+#ifndef WIN32_LEAN_AND_MEAN
 #define WIN32_LEAN_AND_MEAN
+#endif
 #include <windows.h>
 #define HAVE_MMAP 1
 #define HAVE_MORECORE 0