[IMP] base: Add compute/inverse methods to manage user groups
[IMP] base: Add compute/inverse methods to manage user groups

Purpose
=======

Allow to define users groups on a res.user with computed fields based on the 'groups_id' field.

Specification
=============

Add compute/inverse methods to manage specific groups on a res.users.

These methods should be used on fields defined like one the following cases:

1. A boolean field with the compute/inverse method and an attribute group_xml_id.
Example:
    has_group_discount_per_so_line = fields.Boolean(
        "Discount on lines",
        compute='_compute_groups_id', inverse='_inverse_groups_id',
        group_xml_id='sale.group_discount_per_so_line')

2. A selection field with the compute/inverse method, the selection got from '_get_group_selection'
   and an attribute 'category_xml_id'
Example:
    group_sales_team_user = fields.Selection(
        selection=lambda self: self._get_group_selection('base.module_category_sales_management'),
        string="Sales", compute='_compute_groups_id', inverse='_inverse_groups_id',
        category_xml_id='base.module_category_sales_management')

The fields will be set according to the groups defined in the 'groups_id' field, and the 'groups_id'
field will be modified according to the modifications made on theses fields when saving the users's
form.

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