scrolledwindow: Kinetic scrolling support

scrolledwindow: Kinetic scrolling support

Kinetic scrolling is only done on touch devices, since it is
sort of meaningless on pointer devices, besides it implies
a different input event handling on child widgets that is
unnecessary there.

If the scrolling doesn't start after a long press, the scrolling is
cancelled and events are handled by child widgets normally.

When clicked again close to the previous button press location
(assuming it had ~0 movement), the scrolled window will allow
the child to handle the events immediately.

This is so the user doesn't have to wait to the press-and-hold
timeout in order to operate on the scrolledwindow child.

The innermost scrolled window always gets to capture the events, all
scrolled windows above it just let the event go through. Ideally
reaching a limit on the innermost scrolled window would propagate
the dragging up the hierarchy in order to keep following the touch
coords, although that'd involve rather evil hacks just to cater
for broken UIs.