Improve Tooltip accessibility

[TyMick]

I may make this a 20% project at some point, but I think it'd be nice if Tooltip automatically added an aria-describedby prop to the target element pointing to the tooltip itself whenever the tooltip is visible. And maybe when the tooltip isn't visible, it can instead add an aria-label to the target to convey the same information to screen reader users without them having to deal with the popover at all.

Had this idea while writing this tooltip in Faithlife#4639.