Accessibility - provide WAI-ARIA properties for list elements

Accessibility for a virtual list could be lacking. ARIA provides a way to mitigate this; even if it may not be supported by accessibility user agents, it's worth having and doesn't break anything else.

See http://www.w3.org/TR/wai-aria/states_and_properties#aria-setsize - by defining setSize and posInset, an accessibility user agent can know how large the list is despite the entire representation not being in the DOM.

This should only require aria properties on the list items in the html template, but it may also require the `ul` element to be an aria live region.

Start by locating the virtual list's HTML template and inspect the list-item and ul elements. Use the linked WAI-ARIA setsize and posinset guidance to determine the needed properties, and assess whether the ul should be a live region. Done means the rendered list exposes the virtual list size and each item's position without breaking existing behavior.

Written by the indexing model from the issue text.