This will be used to just detect when an edge of the scrollable area is
reached - as opposed to the edge-overshot signal that is emitted when
the user scrolls past the edge.
https://bugzilla.gnome.org/show_bug.cgi?id=742848
This signal is emitted whenever user scrolling hits the overshoot
edge in the given direction. May be useful to add "reload" or "load
more" behaviors in apps.
https://bugzilla.gnome.org/show_bug.cgi?id=738534