custom field types based on reference field

[McAronDev]

Fixes ignoring references when extending reference field classes.
now this works fine :

  var ReferencedListField=nga.getFieldConstructor('referenced_list');

  class CustomReferencedList extends ReferencedListField {
    constructor(name) {
      super(name);
      this._type = 'custom_referenced_list';
    }
  }

[floo51]

Can you try not to break backward compatibility ?