Simplify handling of adjustments

The relationship between adjustments and scrollbars and
scrollable widgets was much more complex than it needed to be.

StScrollView: Have the scroll view own a pair of adjustments,
  set them on the child on add(), remove unnecessary
  change notification signal connections.
StBoxLayout: Remove auto-create of adjustments, just take the
  adjustments from the scrollbars and set them on the scrollable
  child. Notify for hadjustment/vadjustment properties.
StScrollBar: Notify adjustment property.
StScrollable: Document how adjustment setting works.

https://bugzilla.gnome.org/show_bug.cgi?id=611740