[IMP] Order groups by selection declaration order in read_group

[Elkasitu]

Before this commit, calling read_group on a model and grouping by a
selection field would return the groups in alphabetical order, meaning
that if your selection field's options were declared as:

('z', 'Z'), ('a', 'A')

read_group would return first group 'a' and then group 'z', instead of
groups 'z' and then 'a' which some might expect.

With this commit, it is now possible to set a fields.Selection
group_expand attribute to True when declaring it, which will use a
default group_expand implementation specific to Selection fields, this
means that when grouping by a Selection field with group_expand=True
it will always return the groups in the definition order of the
selection options.

We achieve this by leveraging the group_expand field attribute which was
designed for changing the groups returned by read_group.

Since this attribute was thought only to be implemented on a
Model-by-Model basis and here we need to use it as a generic function
for all Selection fields (explicit group_expand declarations have higher
precedence), the generic method has been implemented inside
fields.Selection and takes an extra records parameter which holds a
reference to the recordset/model on which read_group was called. This
extra parameter only applies to field implementations of group_expand
and in this case is what allows this specific feature to work with
dynamic Selection fields (function as options).

A side-effect of this implementation is that a read_group call that
groups by a Selection field with group_expand=True that uses the
default group_expand will always return all possible groups, even
empty ones.

Task-ID 2635052

[robodoo]

Pull request status dashboard.

[rco-odoo]

@robodoo r+