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

src/st/st-scroll-bar.c

@@ -1143,6 +1143,10 @@ st_scroll_bar_set_adjustment (StScrollBar  *bar,
   g_return_if_fail (ST_IS_SCROLL_BAR (bar));
 
   priv = bar->priv;
+
+  if (adjustment == priv->adjustment)
+    return;
+
   if (priv->adjustment)
     {
       g_signal_handlers_disconnect_by_func (priv->adjustment,
@@ -1168,6 +1172,8 @@ st_scroll_bar_set_adjustment (StScrollBar  *bar,
 
       clutter_actor_queue_relayout (CLUTTER_ACTOR (bar));
     }
+
+  g_object_notify (G_OBJECT (bar), "adjustment");
 }
 
 /**