Hi folks, this is Michael Zlatkovsky, your host in the video above.
For thsoe who asking questions in the comments (or otherwise want a reply from me): I actually don't get notified when comments are added, so I only see them once in a blue moon. If you have questions, could you please post them to STACKOVERFLOW instead, and tag them with [office-js]? I'll be more than happy to answer there.
@Glyn, sorry for the delay -- I didn't see that you'd replied to the comment. I think the reason is that in Visual Studio 2015, the default templates now make use of Add-in commands... and for those, there is not designer experience. I would encourage you to file a suggestion on https://visualstudio.uservoice.com/ requesting the feature to be brought back (or have a web-based manifest designer, etc.)