[FIX] sale_margin: stored field functions cannot trigger other stored field functions

the update of sale.order margin field was trigger with the margin stored field of sale.order.line: This cannot work with the old api
[FIX] website*sale: product_id_change qty is 0 by default

Thus, need to pass the actual quantity to product_id_change in order to set values correctly
[IMP] fields: split the default value from the compute function

This separation allows to set a default value on a related field, for instance.