scrolledwindow: Trigger builtin kinetic deceleration on libinput devices

scrolledwindow: Trigger builtin kinetic deceleration on libinput devices

The libinput driver will send a 0/0 scroll event on touchpads and other
devices where it knows scrolling stopped for sure. Use these events to
trigger kinetic scrolling from there.

The mechanism is similar to GtkGestureSwipe, we keep a backlog of the
latest dx/dy till a previous point in time, and calculate the final
velocities from there, with the difference we're dealing with scroll
units, and not pixel distances.

https://bugzilla.gnome.org/show_bug.cgi?id=749770