appDisplay: Ensure we don't recreate existing AppIcons for folders

This was missed in 910037f014, make sure
we do the same thing for AppIcons that are created when reloading
folders.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/851

js/ui/appDisplay.js

@@ -1417,7 +1417,10 @@ class FolderView extends BaseAppView {
             if (apps.some(appIcon => appIcon.id == appId))
                 return;
 
-            let icon = new AppIcon(app);
+            let icon = this._items[appId];
+            if (!icon)
+                icon = new AppIcon(app);
+
             apps.push(icon);
         };