JavaScript addition which allows for custom polling importers to be added to the Add... dialog in the Polling Importer UI

One of the challenges with the Polling Importer UI is that it is non-trivial to add new options to Type field in the Add... dialog. The current options are hardcoded in the file `/libs/cq/ui/widgets/source/widgets/wcm/FeedImporter.js`

With this addition, any `Importer` service which has the special service property `displayName` will get dynamically added to the list.