Autocomplete purchase invoices from receptions

This module adds a helper field on vendor bills so you can build a bill directly from a goods reception instead of typing lines by hand.

• On a draft vendor bill (account.move of type in_invoice), a new field "Auto-Complete from Picking" appears next to the existing purchase-order field. It only lists receptions that are done, incoming, belong to the same company and vendor, and still have something waiting to be billed.
• Choosing one loads the vendor, currency and other header data from the related purchase order, then creates invoice lines for each received product quantity that has not yet been billed. Lines already linked to a stock move are skipped, so nothing is duplicated.
• Each generated line keeps a link to its stock move and to the reception, shown as optional columns on the invoice lines.

Behind the scenes, stock moves and receptions get billing indicators: "Billed Qty", "To Invoice Quantity" and a "Billing Status" on the reception (Nothing to Bill, Waiting Bills, Fully Billed). These are computed from the linked invoice lines, counting vendor bills up and refunds down, and ignoring cancelled invoices.

The module touches account.move, account.move.line, stock.move and stock.picking, and depends on purchase_stock.