Add auto-scrolling to st-scroll-view.

- It must be explicitly turned on via set_auto_scrolling method.
- It defaults to on, selectable in Cinnamon Settings
- Establishes a region to the top and bottom of the StScrollView
  that overlaps slightly in the view itself, and mostly above and
  below the view.
- There are 3 regions - slow, medium, fast
- Only vertical scrolling is supported at this time
- Scrolling stops of your horizontal position moves outside the
  bounds of the scroll view, or you move up or down out of the
  scrolling region
- Initiation of autoscrolling can only happen directly inside the
  scroll view (the extended regions are not sensitive until you've
  triggered autoscrolling within the original view)
st-scroll-view - instead of 3 regions, just scale across the entire
scrolling region, for more granular control of the scroll speed.
st-scroll-view- Remove unused enum.
st-scroll-view - fix math error