Support hiding of all TabPanels

Firstly, thanks for this great lib. The focus on a11y is a huge +1.

The UI I'm aiming for is a tab-strip with all the tab panels closed, and a tab is activated only when the user selects a tab. It looks like react-tabs doesn't currently support hiding all the tab panels.

I searched the issues here to find #287, but this seems to be using a CSS-only solution. This will still cause the DOM to have the tab panel. This not only adds unnecessary stuff to the DOM (my tab panel could have a lot of DOM nodes), but also fires off the rendering lifecycle of the tab panel's contents, including all the hooks for that panel. In my case, this would trigger network requests and such, which I would like to avoid.

Would it be possible to build support for something like defaultIndex={null}, so that all tab panels can be hidden away until needed?

Inizia tracciando il modo in cui react-tabs gestisce defaultIndex e come il rendering di TabPanel reagisce quando non è selezionata alcuna scheda. Definisci il comportamento di defaultIndex={null}, inclusa l’attivazione dopo la selezione di una scheda, e copri con dei test sia lo stato inizialmente vuoto sia la selezione successiva.