Returns the list of selected column names.

def columns(self):
        """
        Returns the list of selected column names.
        """
        fields = [f.label for f in self.form_fields
                  if self.cleaned_data["field_%s_export" % f.id]]
        if self.cleaned_data["field_0_export"]:
            fields.append(self.entry_time_name)
        return fields