Add option to mark a Purchase Order as "Closed"

We very much miss the feature in QuickBooks (which we were using before D-Tools) where we can just mark a PO as "Closed" if we don't anticipate to receive the rest of it or it was cancelled by us or the vendor or any other reason. The backend work of correcting a PO so that it doesn't have to just sit as unreceived or partially received forever within D-Tools seems uneccessary when we could just mark it as closed and accounting can pay invoices and handle the rest from there.