Fixes#29
Upstream Headless UI handles this for Radio Group but not for the others. I didn't see any reason why not to implement this for the other ones too.
When porting, I thought I was being clever by just using ids instead of refs in $api.panels
But I didn't notice that the refs let you distinguish those panels which are rendered from those which are not.
My implementation had aria-controls= set for all tabs, even when their panels were not in the DOM.
