#1417: Fixes problem of dragging hundreds of photos by using XDS instead of standard GTK drag-and-drop when 
dealing with Nautilus.  Because of this change, photo export is remarkably simpler.  Also, refactored the DnD 
export code out to a separate class, and the batch export code to a separate function.  Also, a case where 
dropping 5000 photos onto a tag being excessively slow is fixed.