ControllerInterface: Add a way to register callbacks

This adds RegisterHotplugCallback() to register a callback which will
be invoked by the input backends' hotplug threads when there is a new
device, so that Core (GCKeyboard, GCPad, Wiimote, Hotkey) can reload
the configuration without adding a dependency to Core from InputCommon.

Source/Core/InputCommon/ControllerInterface/ControllerInterface.h

@@ -127,7 +127,11 @@ public:
                        const ciface::Core::DeviceQualifier& default_device) const;
   void UpdateInput();
 
+  void RegisterHotplugCallback(std::function<void(void)> callback);
+  void InvokeHotplugCallbacks() const;
+
 private:
+  std::vector<std::function<void()>> m_hotplug_callbacks;
   bool m_is_init;
   void* m_hwnd;
 };