UI: Trigger scene change event on scene collection load

When the program first starts up, or rather when a scene collection is
loaded, the frontend API OBS_FRONTEND_EVENT_SCENE_CHANGED event would
not get triggered.

UI/window-basic-main.cpp

@@ -961,6 +961,9 @@ retryScene:
 	LogScenes();
 
 	disableSaving--;
+
+	if (api)
+		api->on_event(OBS_FRONTEND_EVENT_SCENE_CHANGED);
 }
 
 #define SERVICE_PATH "service.json"