Pin a website/web app with no manifest

[benfrancis]

As a user I want to pin a website/web app which doesn't provide a web app manifest, so that I can use it in an app-like window with no address bar.

Currently Shell only allows users to pin web apps if they provide a web application manifest.

Following a change in direction by major browser vendors, the W3C Web Applications Working Group are working on a new definition of installable web applications which means that any website could potentially be an installable web application.

Currently this idea depends on the concept of a "default manifest" which is assumed by some implementations but is not yet specified in the Web Application Manifest specification.

This feature would mean it is possible to pin any website as an app, not just those that link to a web app manifest, by deriving default values for start_url, id and scope from the document URL, and perhaps falling back to other metadata for application name and icon.