Descriptions for interactives

[sean-fitzpatrick]

The pull request in #2569 adds infrastructure for both short and long descriptions of Asymptote images.

These are handled differently than latex-image since 3D Asymptote images live in iframes.

Since interactives also live in iframes, similar code changes should work for them, to at least describe the static version of an interactive element.

One note: in #2569 I made the decision to write the short description into the title attribute of the iframe. Another option would be to dump it into the body of the iframe, like

<iframe attributes>
alt text
</iframe>

rather than a self-closing tag. I tried finding opinions on which is better for accessibility, but didn't find any.

The latter option would cause the alt text to appear on the page in an event where the iframe failed to load.

[sean-fitzpatrick]

According to this site the title is read by screen readers, so I think the current change for Asymptote is what we want.