[Icons]: improve a11y capabilities

[AlanBreck]

As of now, we don't accept any attributes on <Icon> to allow consumers to improve accessibility of the icon. I'm not entirely clear on the ideal approach for a11y with icons, but I've seen two approaches put forward:

1. Place aria-hidden="true" on the icon and provide a visually-hidden text element alongside it.
2. Add a title attribute to the icon.

I personally prefer option 2, but it wasn't as frequently mentioned as a solution in my brief search.