[w3c/manifest] Standardize members for "screenshots" member (#947)

See https://twitter.com/mariusclaret/status/1350764987556519937 for context (request).

Browsers expose more fields (including screenshots) from web app manifest to PWA install prompt. See https://twitter.com/firt/status/1350161036897300484 for implementation in Chrome on Android. I expect the same for desktop.

So there is a need to distinguish screenshots by platform to display e.g. only mobile screenshots on mobile, desktop screenshots on desktop, etc.

I've seen `"platform"` and `"label"` for screenshots in https://sadchonks.com/manifest.json, but it seems they are not in the spec.

(IMO, `"description"` instead of `"label"` is a better name for text that is displayed on screenshot hover.)

Is the [`"purpose"`](https://w3c.github.io/manifest/#purpose-member) member (with `"mobile"`, `"tablet"`, `"desktop"` values) valid member of "screenshots" member?

Anyway, I believe that `"purpose"` member alone isn't enough. E.g. there are Andoid phones and Android tablets. `"platform"` is more about OS (`"xbox"`, `"android"`, etc.), `"purpose"` is more about form-factor.

BTW, currently, there is the `"platform"` member for `"related_applications"`, but it represents a software distribution ecosystem (`"play"`, `"itunes"`, `"chrome_web_store"`, etc,) (see https://github.com/w3c/manifest/wiki/Platforms).

So maybe it makes sense to name them `"os"` and `"form_factor"`?

Also, what about optional `"orientation"` member with (`"landscape"` and `"portrait"` values) for screenshots?

This should also helps app stores opened for PWAs (Samsung Galaxy Store, Microsoft Store, etc.) to check requirements and publish PWA by manifest URL.

E.g, here are the Google Play’s screenshot requirements:

> * If you have an app for different devices (think: Tablet, Android Wear, etc.) you must upload screenshots specific to each device
> * Images must be in JPEG or 24-bit PNG
> * A minimum of 2 screenshots is required
> * Minimum dimension: 320px, maximum dimension: 3840px

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