Add auto-scrolling to st-scroll-view.

- It must be explicitly turned on via set_auto_scrolling method.
- It defaults to on in the Menu applet, 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.
- Speed scales across entire region of scrolling from very slow to very 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)