Accessibility: indicate which thumbnail is selected, for screenreaders

[lhenze]

This issue is a subticket of #3578
It concerns the last bullet point,
Thumbnail panel at the bottom lack aria-selected.

See: https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-selected

Since the element in question has role gridcell , I agree that adding aria-selected to the selected thumbnail makes sense.

So for each div in the thumbnail area where role is gridcell , let's add the aria-selected attribute, set to true or false as appropriate.