Implement support for "progress payment invoices" (Abschlagsrechnung)

[dragotin]

Often, after a construction started for example, a "Abschlagsrechnung" is sent to make the client pay for material that was delivered, but the project is not finished yet. It is about a pauschal sum that is later subtracted from the final invoice.

User story: The user clicks on an "Offer/Quote (Angebot)" or "Acceptance of Order (Auftragsbestätigung)" and selects "Followup Document". In the following dialog, the doc type for "Progress Payment Invoice" is selected. A normal document is created.

Later, the user clicks again on the "Offer" and wants to create the final invoice. Now, in the following dialog, there is a hint that there is one or more "Abschlagsrechnung" for this doc. Once the user creates the final invoice, an item for each "Abschlagsrechnung" is created automatically with the negative sum of it.

Database Migration steps:

• Create the new doc types Progress Payment Invoice, Partial Invoice and Final Invoice
• Create attributes for the new doc types: SubstractPartialInvoice for the Final Invoice and PartialInvoice for Progress Payment Invoice, Partial Invoice.
• Add a column predecessor to the document table to track the document predecessor (store Document Ident)
• Create predecessor and successor definitions for the Doctypes, ie. Partial Invoice follows on Offer and Final Invoice follows on Partial Invoice.

When creating the final invoice as a predecessor of document ident XY, a query needs to be done if there are other documents with a doc_type that has the attribute PartialInvoice and the predecessor == XY. For all found documents, an item has to be created in the final invoice doc, with the netto sum of the partial invoice.

[dragotin]

This is already available from version 0.90.