Move Tweener.slowDownFactor into St

It has probably crossed the line to evil by a mile or so, but here
it is: a Tweener.slowDownFactor replacement used by all animations
without exception.
While at it, update Tweener to use the new setTimeScale() upstream
function instead of adjusting the timeline directly.

https://bugzilla.gnome.org/show_bug.cgi?id=622249

src/st/st-widget.c

@@ -105,6 +105,8 @@ enum
 
 static guint signals[LAST_SIGNAL] = { 0, };
 
+gfloat st_slow_down_factor = 1.0;
+
 G_DEFINE_ABSTRACT_TYPE (StWidget, st_widget, CLUTTER_TYPE_ACTOR);
 
 #define ST_WIDGET_GET_PRIVATE(obj)    (G_TYPE_INSTANCE_GET_PRIVATE ((obj), ST_TYPE_WIDGET, StWidgetPrivate))
@@ -1704,3 +1706,27 @@ st_describe_actor (ClutterActor *actor)
 
   return g_string_free (desc, FALSE);
 }
+
+/**
+ * st_set_slow_down_factor:
+ *
+ * @factor: new slow-down factor
+ *
+ * Set a global factor applied to all animation durations
+ */
+void
+st_set_slow_down_factor (gfloat factor)
+{
+  st_slow_down_factor = factor;
+}
+
+/**
+ * st_get_slow_down_factor:
+ *
+ * Returns: the global factor applied to all animation durations
+ */
+gfloat
+st_get_slow_down_factor ()
+{
+  return st_slow_down_factor;
+}