Bug 995035 - Hide invisible parts of SMS app from screen reader.

Actually hiding the elements (as opposed to aria-hidden) gives a better user experience as the accessible tree state changes and the user is notified by having the cursor moved.

aria-hidden, could be used as a last resort, but I am also concerned with the complexity of the navigation.js code, and not getting all cases right. Doing it in CSS assures that we are consistent with the visual state.