st: Move slow-down-factor into settings

Now that we have a Settings singleton, we have a better place for the
slow-down-factor than an awkward extern variable.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/656

js/ui/tweener.js

@@ -211,8 +211,9 @@ var ClutterFrameTicker = class {
     }
 
     start() {
-        if (St.get_slow_down_factor() > 0)
-            Tweener.setTimeScale(1 / St.get_slow_down_factor());
+        let settings = St.Settings.get();
+        if (settings.slow_down_factor > 0)
+            Tweener.setTimeScale(1 / settings.slow_down_factor);
         this._timeline.start();
         global.begin_work();
     }