Develop SourcePath class
Implement SourcePath validation
Test OrderLineSourcePathValidator
Add Total properties to PurchaseOrderService
Refactor PurchaseOrderService
Rename PurchaseOrderService to InvoiceService
Remove TextBox from UI

Remove unneeded TextBox from UI, and move DragDrop trigger to the
window. Remove SourcePath property from ShellViewModel.
Integrate TotalsView into UI