title: Get Prefab file guid at runtime
body: <p>We use JSON to save some user settings in our game. We want to persist the users inventory between game sessions so he do not need to reconfigure his inventory every time.</p>
  
  <p>For this I need to get a Gameobjects Prefab GUID (All of our items that you can have in inventory are Prefabs) and on next game session be able to instance correct Prefab. I have looked at the methods in <em>PrefabUtility</em>  but cant find anything I can use there. So any tips please.</p>