code: def get_extra_data(self, data):
          
          result = {}
          for key in data.keys():
              if key.startswith(self.name + '-'):
                  new_key = key.replace(self.name + '-', '')
                  result[new_key] = data[key]
          return result
docs: Get eventual extra data for this placeholder from the
          admin form. This method is called when the Page is
          saved in the admin and passed to the placeholder save
          method.