Refactor the grouping code so that it can be extended.

#992704: Group by field also groups by entity_id exposed the need to extend the group by settings form in order to allow an additional setting required for the Field API fields to be grouped on, then have our own group by code which takes that setting into account.

However, the group settings form is rendered from views_ui_config_item_group_form() in admin.inc, instead of being located in a handler (views_handler_field, for instance).
Additionally, the query itself is modified in views_plugin_query_default.
This means that the grouping functionality is not overridable, even though it would work perfectly well to do it all from handlers, making it overridable.

Documenting this so that we can give it attention later on.
Making it for 6.x-3.x since there's no reason not to apply the changes there too.