FieldStatus was broken:
- With a selection, the statusbar_visible attribute forces the choices
to display, the discarded ones do not appear at all (even folded)
- With a m2o, the fold_field option tells the field to check in the
relation to know if the choice has to be folded or not, this was not
properly supported
- Changing a m2o value was broken as the parseMany2one function did
not exist
- The status were misordered
- ...
* Also refactor the way the rendering is handled.
* Also modify all statusbar tests so that the statusbar widget is
placed in the form header tag (as it is the common use case).