[IMP] Extend fields.Selection with db_index_type parameter. It can use

to create an empty selection, to fill it up later with the selection_add
parameter (from modules for example).

The Postgres db handler need to know which kind of index field to
create, varchar or integer, so if you create an empty fields.Selection
you need to define the database fields type to create.
Accepted values for db_index_type are 'str' or 'int'.

Samples:
    srv1 = fields.Selection([], string='Services', db_index_type=int)
    srv2 = fields.Selection([], string='Services', db_index_type=str,
size=4)

Bug #3810