Added item rotation, vertical movement and stable stacking so that you can keep things nice and tidy. You don't have to, but items might spill out once it gets too full. While dragging the item, left/right arrows rotate the item and up/down moves it vertically.
One advantage of this system is that you can just stuff an item into a specific spot and it will push the others out of the way. I will add subcontainers and some way to auto sort by category to help keep things organised.