This commit proposes to store the complete name of a product category.
It is formatted like parent.name / name, recursively. Storing it and
using it as a rec_name allows to search products belonging to a category
or its child categories without having to rely on a strange hack of
name_get and name_search.

Searching on products having a category containing "all" will get results
from "all" category as well as "all / saleable", and so on.