* Don't just use the domain out of a FilterGroup as group_by keys in filters will overwrite one another
* Add dedicated get_groupby API returning an array of groups to evaluate
limitation: can't intersperse groupby from a different FilterGroup, that'll have to be fixed by ungrouping buttons
bzr revid: xmo@openerp.com-20120403120742-o3liksqaigsnxvb4