Monkey-patch autoform to add an hidden input on the main form (we can't use
'formtype' since it's not appropriate (may be either 'main' or 'inlined'),
rather check if 'peid' is in kwargs to detect if this is the root form or a sub
form.

Then, propagate value found there as a form param when calling the ajax function
to render an inline creation form. Overriding is not necessary for edition form,
since in that case we already have the information.

Doing so allows to simplify the parent_and_container implementation, and some
place are we can now expect the container to be always detectable.