Adds the ability for the Purchaser to specify where the item is
intentended to go when received. If the Purchaser does not set a
preferred location, then the default location for the part is displayed.
If the item is received them where it was actually placed is shown.
NOTE: if an item is split when received only one of the resulting
StockItem location is used.
Fixes#1467
Addresses some of the requests in #551
Stock items show the PO number that they were purchased on when being
viewed in the sales order allocation modal and when viewing the sales
order details.
Nigel