dash: Use the correct theme node for spacing/padding

We consider spacing and padding in _adjustIconSize, but as we use
the theme node from an actor which is not exposed to the CSS, we
miss the "real" values - correct this.

https://bugzilla.gnome.org/show_bug.cgi?id=662213

js/ui/dash.js

@@ -501,7 +501,7 @@ const Dash = new Lang.Class({
             return;
 
 
-        let themeNode = this.actor.get_theme_node();
+        let themeNode = this._box.get_theme_node();
         let maxAllocation = new Clutter.ActorBox({ x1: 0, y1: 0,
                                                    x2: 42 /* whatever */,
                                                    y2: this._maxHeight });