Allow aria-hidden=true on picture element

[scottaohara]

presently the picture element allows for no roles or aria-* attributes, which generally makes sense because the picture element itself is not an accessible object. That said, there could be a situation where a picture that contains a decorative image needs to be hidden..... and for whatever reason the expected alt="" on the fallback image cannot be used, but aria-hidden=true on the picture element could. Upon testing this just now, seems like this would be a viable option as Chromium browsers, Firefox and Webkit all ignore the child <img> in this situation.

[stevefaulkner]

WFM