Override default DX add form to obtain renamed IDs correctly

[ramonski]

Description of the issue/feature this PR addresses

This PR fixes a side-effect of the portal-catalog patch introduced in #2368

Current behavior before PR

DX contents could not be created and failed with the traceback:

Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 176, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 385, in publish_module
  Module ZPublisher.WSGIPublisher, line 288, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZPublisher.WSGIPublisher, line 63, in call_object
  Module plone.z3cform.layout, line 63, in __call__
  Module plone.z3cform.layout, line 47, in update
  Module plone.dexterity.browser.add, line 138, in update
  Module plone.z3cform.fieldsets.extensible, line 65, in update
  Module plone.z3cform.patch, line 30, in GroupForm_update
  Module z3c.form.group, line 145, in update
  Module plone.app.z3cform.csrf, line 22, in execute
  Module z3c.form.action, line 98, in execute
  Module z3c.form.button, line 315, in __call__
  Module z3c.form.button, line 170, in __call__
  Module plone.dexterity.browser.add, line 116, in handleAdd
  Module z3c.form.form, line 265, in createAndAdd
  Module plone.dexterity.browser.add, line 95, in add
AttributeError: 'NoneType' object has no attribute 'id'

Desired behavior after PR is merged

DX contents can be created

--
I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.

[xispa]

Thanks for the quick fixture