Add the necessary animations to slide in the icons in the previous/next
pages, also needing to 1) drop the viewport clipping, and 2) extend scrollview
fade effects to let see the pages in the navigated direction(s).
The animation is driven via 2 adjustments, one for each side, so they
can animate independently.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1630>