Replace `Panel` `aria-disabled` attribute with `data-disabled` (#698)

Both `Panel` and `Separator` were previously rendering with
[`aria-disabled`](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Reference/Attributes/aria-disabled)
when disabled. In the case of `Panel`, this had an unintentional
secondary effect of disabling elements within the panel.

This commit replace that attribute with another one, `data-disabled`,
since this library depends on being able to check _some_ attribute for
disabled state at runtime.

Resolves #697