[IMP] web: search panel counter changes

This commit introduces several changes in the search panel with
respect to record counts:

    - the record counts are now also available for
      the fields with select="one" attribute
      (if not disabled explicitely).
    - the record counts are better computed using the idea that
      selected values within a group should not impact the counts
      for the group values but only the counts for the other
      group values.

On the way we have changed two keys in the values returned
by the server:
    - 'count' becomes '__count'.
      It has been done to avoid a possible clash in case a model would
      have a field named 'count' and that the field values would be
      wanted for some reason.
    - 'name' (multi case) becomes 'display_name'.
      It has been done in order to make the select one and multi cases
      more similar and factorize some code.

TASK-ID: 2166814

Co-authored-by: Raphaël Collet <rco@openerp.com>
Co-authored-by: Mathieu Duckerts-Antoine <dam@odoo.com>
Co-authored-by: Alexis Lacroix <laa@odoo.com>
Co-authored-by: Julien Mougenot <jum@odoo.com>