Fix missing `plone.autoform` ExportImportHandler

[petschki]

With this you can customize widget attributes inside the <form:widget /> node ...

<?xml version="1.0" encoding="utf-8"?>
<model xmlns="http://namespaces.plone.org/supermodel/schema"
       xmlns:form="http://namespaces.plone.org/supermodel/form"
       xmlns:i18n="http://xml.zope.org/namespaces/i18n"
       xmlns:security="http://namespaces.plone.org/supermodel/security"
       i18n:domain="collective.z3cform.datagridfield"
>
  <schema>
    <field name="table"
           type="zope.schema.List"
    >
      <title>Tabular Field</title>
      <description />
      <value_type type="collective.z3cform.datagridfield.row.DictRow">
        <schema>collective.z3cform.datagridfield.demo.interfaces.ITableRow</schema>
      </value_type>
      <form:widget type="collective.z3cform.datagridfield.datagridfield.DataGridFieldWidgetFactory">
        <allow_reorder>True</allow_reorder>
        <auto_append>False</auto_append>
      </form:widget>
    </field>
  </schema>
</model>