Avoid desktop coordinates where possible

Inkscape's API uses "desktop coordinates" in too many places, "document coordinates" would often be more appropriate. "Desktop coordinates" require additional information (available from SPDesktop::doc2dt - might change with #340 (closed)).

Examples:

• getItemsInBox and getItemsPartiallyInBox (done !840 (merged))
• sp_export_png_file (done !840 (merged))
• SPItem::getCenter