Aria prop inconsistencies

[mxstbr]

While writing the TypeScript definitions for all the components (#490), I noticed inconsistencies in the names of aria props.

These all set the aria-label attribute on the rendered element:

• TextInput.aria-label
• Tooltip.text
• UnderlineNav.label

It would be good if these were internally consistent with one another. I think the prop should be called aria-label across the board, as that's the clearest.

We could also make that change in a backwards-compatible manner, so that no existing apps built with these components are broken.

What do you think?

[emplums]

This sounds good to me!