[w3c/manifest] Please add caption property to screenshots (#773)

Since `screenshots` is intended to be used by app stores, and because all app stores use textual captions associated with each screenshot, we need to add a `caption` (or perhaps `description`) string property to each screenshot object.

As it stands right now, you can just define a bunch of screenshots that will get displayed in an app store without any context or description.

Of course, `caption` can be an optional property.

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