BUG: use a mini scene for volume loading

The volumes logic was adding in multiple volume,
display, storage nodes as it tries to load a file. This
generated lots of node added/removed events so it was placed
inside of a batch processing state for the scene. This leads
to no node added event after the batch processing end
event so the warning about adding new storable nodes
to the scene after a scene view is already present wasn't
getting triggered.
This change uses a mini test scene to try loading data
(borrowing the main scene cache manager in case the data is remote)
and then adding the successful set of volume nodes to the main
scene, with an extra node added event after batch processing ends.

Issue 0003956

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24383 3bd1e089-480b-0410-8dfb-8563597acbee