Add change log support to the web app manifest

[stanleyhon]

Native applications typically have a way to show users it was recently updated - and for users to click through and see what's new. I'm proposing a new optional subtree in the web app manifest that allows web app developers to provide this info.

I've started a discussion on WICG here. It includes a link to the Microsoft Edge explainer.

As a summary here, the idea is that a PWA developer adds a URL under the changelog subtree of the web app manifest and a version value. The user agent/platform based on these two values will decide if a user should be notified of a new changelog, and provide a way to see what is pointed at by the URL.

This feature is proposed specifically to not interfere with w3c/manifest#446. For more information, see existing issues on the Microsoft Edge Explainers issues page.

[marcoscaceres]

As we discussed elsewhere, I think we need to perhaps split out store specific metadata and stuff that the browser itself would provide. Few issues with the proposal were around the formats, and the display of those formats.

@stanleyhon, happy to discuss further. Let us know how you want to proceed.

[aarongustafson]

I feel like this should move to App Info. Thoughts?