[forms] propagate aguments given to .vocabulary to .choices if it's a callable. So we can properly uses vocabulary(limit=xxx) in places where we used to call .choices(limit), since field.choices is not meant to be accessed directly. Also, document expected behaviour for custom choices method for relation fields.

reorganize code: move everything related to inlined forms and generic relation from editviews/editforms where there are used, eg in autoforms

[forms] fix backward compat for field.choices arguments