[FIX] Propagate changes in invoice partner or company to invoice lines' related fields

The related fields 'partner_id' and 'company_id' in account.invoice.line are stored but they do not have update trigger. If the invoice partner or company get updated, the invoice lines keep old values. As a result,
these fields are totally unusable and may cause difficult to find bugs in any customization that use them.

This commit adds a `store` update trigger to these fields.