More categories for product
For now, the product is limited to one category (instead of many) because it is used in account_product to get accounting values (which should be unique).
But categories could be used in the same way as the category of party.
So the idea is to add a categories field and to change the current category into an account_category.
Another point is that user will create more category trees which will not have accounting property so we need a way to distinct category usable for accounting and those who are not.
We could ask why not split the Model into 2 Models? This was the first idea but I think in such case user who want to have the same tree structure for accounting and "marketing" will have to duplicate it. And also the migration of such split will be very complex.