Show returns on stock pickings

This module makes it easier to follow the full return chain of a warehouse transfer (a stock picking).

On any transfer form, a new "Returns" tab appears in the notebook. It lists the return transfers that were created from the current transfer, so you can see at a glance which returns belong to it. The tab is hidden when there are no returns, so it does not clutter the screen.

When the transfer you are looking at is itself a return, a smart button labelled "Source picking" appears in the top button box. Clicking it opens the original transfer that this return was made from. The button is hidden when there is no source transfer.

Behind the scenes, the module adds two computed fields on stock.picking: returned_ids (the return transfers linked to the current one) and source_picking_id (the original transfer, if any). These are calculated automatically from the moves and their return links, so no manual data entry is needed.

No new menus, reports, emails or scheduled actions are added. The module only extends the existing transfer form view and the stock.picking model.