#1417: Fixes problem of dragging hundreds of photos by using XDS instead of...
#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.
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