Core: Make AddOnStateChangedCallback use HookableEvent.

Source/Core/DolphinQt/Settings.cpp

@@ -50,7 +50,7 @@ static std::unique_ptr<QPalette> s_default_palette;
 Settings::Settings()
 {
   qRegisterMetaType<Core::State>();
-  Core::AddOnStateChangedCallback([this](Core::State new_state) {
+  m_core_state_changed_hook = Core::AddOnStateChangedCallback([this](Core::State new_state) {
     QueueOnObject(this, [this, new_state] {
       // Avoid signal spam while continuously frame stepping. Will still send a signal for the first
       // and last framestep.

Source/Core/DolphinQt/Settings.h

@@ -236,6 +236,8 @@ private:
   std::shared_ptr<NetPlay::NetPlayServer> m_server;
   Common::EventHook m_hotplug_event_hook;
   Config::ConfigChangedCallbackID m_config_changed_callback_id;
+
+  Common::EventHook m_core_state_changed_hook;
 };
 
 Q_DECLARE_METATYPE(Core::State);