Add aria-labels for all relevant widgets

[bitspittle]

See also: https://dequeuniversity.com/rules/axe/4.8/label
and #538

Important

Be sure to give user carlosgub credit for when this goes in.

[bitspittle]

This is probably something we want to get in before 1.0 -- but at the very least, we should look into this more and, if we decide we can punt to 1.1, then we should add a comment explaining why.