if you pass a `selectedIndex` prop, then that tab will be selected, no
matter what. it doesn't change automatically from user interaction. to
change in response to interaction, have the parent pass an `onSelect`
handler that updates the `selectedIndex` prop.

previously, in order for focus management to work, we assumed the parent
would rerender the tabs component when its onSelect handler was invoked.
that was probably a pretty safe assumption, but we no longer rely on it.

also made it more explicit (and enforced) that `selectedIndex` and
`onSelect` come as a pair. and improved test coverage around this stuff.