Separated joystick state from window system state.

This is partially in preparation for pending support for additional
joystick APIs like XInput, DirectInput and IOHID.

src/internal.h

@@ -349,6 +349,8 @@ struct _GLFWlibrary
     _GLFW_PLATFORM_LIBRARY_WINDOW_STATE;
     // This is defined in the context API's platform.h
     _GLFW_PLATFORM_LIBRARY_OPENGL_STATE;
+    // This is defined in the platform's joystick.h
+    _GLFW_PLATFORM_LIBRARY_JOYSTICK_STATE;
     // This is defined in the platform's tls.h
     _GLFW_PLATFORM_TLS_STATE;
 };