viewSelector: Don't show pages until they need to be visible

AppDisplay queues a deferred work to load frequently used apps when the
apps page is loaded. Unfortunately, when the overview is first opened,
all the pages start out visible and then immediately get hidden, so the
deferred work runs immediately after the first overview opening, whether
the user was going to view their frequent apps or not.

Start all pages off as hidden, and rearrange the code so that pages are
only shown when they really need to be.

https://bugzilla.gnome.org/show_bug.cgi?id=712753