When DnDing an icon, we show both previous/next page, and optionally
a "placeholder" actor to allow creating new pages. These sides on the
scrollview are drop targets themselves, allowing to drop an app onto
the next/prev page without further navigation.
Still, preserve the checks to maybe switch to prev/next page without
finishing the DnD operation, for finer grained operations.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1630>