turn all the stuff used to handle 'generic relations' in forms into proper
field / widget.
-> regroup code from web.request, web.controller, web.views.autoform, web.views.formrenderers,
web.views.editcontroller (!) into GenericRelationsField, GenericRelationsWidget in the editviews
module (together with the UnrelatedDiv view).
So:
* almost everything in one place
* no more specific behaviour in the form renderer
* almost no custom behaviour in autoform (simply add the field when it think it should)
Also, the form renderer now display field's value with colspan=2 when field.label is None.