add new W3C Manifest so Marketplace can be added to homescreens

read https://bugzilla.mozilla.org/show_bug.cgi?id=1084548#c0

I also added other tags to support other platforms' legacy "Add to Homescreen."
> Do we even need the hosted manifest anymore?
can remove. considering how Manifest support has landed in Chrome only, not Firefox yet, it might be useful to keep hosted manifest here for now. but can remove in separate commit.

> Are the old paths going to 404 ? Are they used anywhere important ?
very good point. I would keep them in `src`, but Zamboni can't reach them in there; so maybe a soft link?