Fixed: (PurchaseItem)
* show sum in shopping cart
* show id in shopping cart
* increase quantity when adding same product
* "Add to cart" button only enabled when product is selected
* show error when trying to add more products then available in warehouse
* default purchase amount set to "" (equals 1)
* added first blank dropdown item
* warehouse quantity is decreasing when added to shopping cart
* warehouse quantity is increasing when order is canceled
* SoldItem.java added this.id=stockItem.getId()
* can't input negative amount (equals 1)