[FIX] fields: generalize inverse_field to a list of inverse fields

This fixes an issue in website_sale_options: when adding a product with an option in the cart, the total price does not take into account the line with the option.

The cause of the problem is the fact that model sale.order has *two* one2many fields for the sale order lines; the second field has a domain that the first one has not. When a line is created and its many2one is set, only one of the one2many fields is invalidated in cache.

The fix ensures that when a relational field is updated, all its inverse fields are invalidated.