Access DebugMenu from anywhere in the app

The application may not have a key window yet if this is called before the window is setup. The best solution I have come up with in the past is to add the shared instance to `NSNotificationCenter` upon init to observe `UIApplicationDidFinishLaunching`. Then, in the notification handler, you can create the gesture and attach it to the key window, because it should exist and be correctly set to the app's window at that point.