BUG: Fixed volume rendering presets issue (presets were not displayed)

The scene model RowCache was temporarily outdated during node insertion. If the cache was accessed during node insertion, the qMRMLSceneModel::indexFromNode method indicated that the node doesn't exist yet.
Fixed by adding an entry to the cache with the node-to-be-added, which results in not using the cache to determine if the node exists or not but uses the traditional way of iterating through all the items.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22685 3bd1e089-480b-0410-8dfb-8563597acbee