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/filesystem/windows/SDL_sysfilesystem.c

@@ -172,4 +172,23 @@ SDL_GetPrefPath(const char *org, const char *app)
 
 #endif /* SDL_FILESYSTEM_WINDOWS */
 
+#ifdef SDL_FILESYSTEM_XBOX
+#include "SDL_filesystem.h"
+#include "SDL_error.h"
+char *
+SDL_GetBasePath(void)
+{
+    SDL_Unsupported();
+    return NULL;
+}
+
+char *
+SDL_GetPrefPath(const char *org, const char *app)
+{
+    SDL_Unsupported();
+    return NULL;
+}
+#endif /* SDL_FILESYSTEM_XBOX */
+
+
 /* vi: set ts=4 sw=4 expandtab: */