[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.

Some many2one fields happen to have several corresponding one2many fields,
typically with different domains. It is also the case for the field 'res_id' of
mail.message, where each model inheriting from mail.thread defines a one2many
based on that field. The fix ensures that when a relational field is updated,
all its inverse fields are invalidated.