windowManager: Update animation of attached modals (again)

The fade animation we started using after centering attached
modal dialogs didn't work too well. So after going back to the
scale animation, adjust it to scale from the center rather
than the top, which works quite well.

https://bugzilla.gnome.org/show_bug.cgi?id=681601

js/ui/windowManager.js

@@ -301,6 +301,7 @@ const WindowManager = new Lang.Class({
             this._checkDimming(actor.get_meta_window().get_transient_for());
             if (this._shouldAnimate()) {
                 actor.set_scale(1.0, 0.0);
+                actor.scale_gravity = Clutter.Gravity.CENTER;
                 actor.show();
                 this._mapping.push(actor);
 
@@ -377,6 +378,7 @@ const WindowManager = new Lang.Class({
             }
 
             actor.set_scale(1.0, 1.0);
+            actor.scale_gravity = Clutter.Gravity.CENTER;
             actor.show();
             this._destroying.push(actor);