- prefabs need to be stored in Resources/Prefabs

Workflow:
- Window --> XmlLevelEditor

- Delete
- Click "Export..."
- Add all Scene objects as Childs to new Export-Object in Hierarchy (every Object needs to have name of prefab)
- Click "Export..." again
- new Level appears in XmlItemsToExport.xml
- copy Level to Levels.xml

- Load Level from DeserializedLevelsLoader