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.h

@@ -141,8 +141,10 @@ void                  st_widget_set_direction             (StWidget        *self
 void                  st_widget_style_changed             (StWidget        *widget);
 StThemeNode *         st_widget_get_theme_node            (StWidget        *widget);
 
-/* debug method */
-char *st_describe_actor (ClutterActor *actor);
+/* debug methods */
+char  *st_describe_actor       (ClutterActor *actor);
+void   st_set_slow_down_factor (gfloat factor);
+gfloat st_get_slow_down_factor (void);
 
 G_END_DECLS