#2111: Refactoring of BatchImport to reduce the overhead of managing background jobs and their completion 
callbacks, to keep the UI responsive during an import while (attempting) to not sacrifice speed.  There remains 
a corner case when a large number of small files (8,000 8K JPEGs in my test case) will still swamp the UI, but 
we will live with that for now ... importing that many in a responsive way will require more work, and more 
consideration of BatchImport's requirements.