aggregateLayout: Fix natural width computation

Ouch, this went unnoticed for a long time: As the minimum size of menu
items is generally small (because its label can be ellipsized), we are
requesting the unellipsized width of the last "size child" instead of
the widest one.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/996

js/ui/panel.js

@@ -729,7 +729,7 @@ class AggregateLayout extends Clutter.BoxLayout {
             let child = this._sizeChildren[i];
             let [childMin, childNat] = child.get_preferred_width(forHeight);
             minWidth = Math.max(minWidth, childMin);
-            natWidth = Math.max(minWidth, childNat);
+            natWidth = Math.max(natWidth, childNat);
         }
         return [minWidth, natWidth];
     }