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."
yeah, we can do that now with ```TEMPLATE=server make serve```. But wonder if we can reduce the redundancy with that and dev.html (maybe use gulp-inject).