The group base.group_user is used in the computation of the share column which
is executed at column creation, before the group is imported.
Set it in base.sql to already have an xmlid to resolve.
This removal has led to a necessary refactoring:
- make the setup of field attributes extensible;
- make the instantiation of custom fields extensible;
- delegate model and field reflection to `ir.model` and `ir.model.fields`;
- move the implementation of sparse and serialized fields to module `base_sparse_field`;
Raphael Collet