Added undo/redo for transferring items.
I added some javadoc and fixed a few bugs.
Fixed a bug where transferring a product merely deleted it if it had no items.
Fixed a bug involving dragging items around, and refactored a bit regarding dragging products around.
Merge branch 'master' into Reports
I also fixed a bug in product statistics report that made it keep rewriting the first row instead of filling the table.

Conflicts:
	src/model/Item.java