Add a private API that lets widget opt-in to animated updates of
the adjustment value. When enabled, all calls to
gtk_adjustment_set_value will smoothly transition from the old
value to the new value, using a fixed easing function and a
configurable duration. The animation is tied to the frame clock
of the widget.

By implementing this in GtkAdjustment, we can enable animation
for both scrollbars and keybindings, which are often implemented
in the child widget of the scrolled window.

https://bugzilla.gnome.org/show_bug.cgi?id=732376