Expose features' relation to layers? (hierarchies)

[Malvoz]

If each mapml-layer has role="group" and aria-label="{layer name}" it is announced before the featurecaption for the first feature that is focused in a layer, meaning that screen reader users can tell which layer a feature belongs to.

Is this useful? I think so, see motivation in #535 (comment).

[Malvoz]

I suppose visual users can't easily tell the relation unless they toggle the visibility of the layer, but that'd be a separate issue? Maybe relating to (exposed visually in) the feature index?

[prushforth]

I think this makes a lot of sense. Let's do it. I think we should add the layer name to the popup somehow, primarily for visual users.