Make AdwViewStack and AdwTabView accessible

CC @tyrylu

Question: do leaflet and squeezer need the same? They don't have view switchers and just use pages for api purposes - so do they currently work? E.g. libadwaita demo has a leaflet on the toplevel and squeezer - view switcher demo/preferences window.

To me it seems they are working fine atm, but I might be missing something.