Preview gadget with standalone launcher

I have a testing gadget handy:

https://github.com/mereskin/hello-world-gadget/

Use it or use any other iframe gadget out there. Be sure to fire 'startListening' event. Refer to this gadget-api-spec:

https://github.com/Versal/gadget-api-spec/pull/22

API refactoring commit can be extracted into a separate PR, maybe I will do that later. Maybe not.