invoice-list-view: Create an Add to Purchase Order button

This replaces add on close, and adds a new signal add_activated.
purchase-order-editor: Connect to Add to Purchase Order button

Connect to add_activated signal of InvoiceListView object and add the
invoice when emitted.
purchase-order-list-view: Create a Set for Job Order button

This replaces set on close, and adds a new signal set_activated.
job-order-editor: Connect to Set for Job Order button

Connect to set_activated signal of PurchaseOrderListView object and set
the purchase order when emitted.