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/misc/windows/SDL_sysurl.c

@@ -24,6 +24,14 @@
 
 #include <shellapi.h>
 
+#if defined(__XBOXONE__) || defined(__XBOXSERIES__)
+int
+SDL_SYS_OpenURL(const char *url)
+{
+    /* Not supported */
+    return SDL_Unsupported();
+}
+#else
 /* https://msdn.microsoft.com/en-us/library/windows/desktop/bb762153%28v=vs.85%29.aspx */
 int
 SDL_SYS_OpenURL(const char *url)
@@ -49,6 +57,7 @@ SDL_SYS_OpenURL(const char *url)
     WIN_CoUninitialize();
     return (rc > ((HINSTANCE) 32)) ? 0 : WIN_SetError("Couldn't open given URL.");
 }
+#endif
 
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