Use dedicated icons for top bar/windows in ctrl-alt-tab

The previously used ones were quite a stretch, so now that we got
dedicated ones, let's use them.

js/ui/panel.js

@@ -752,7 +752,7 @@ const Panel = new Lang.Class({
         }));
 
         Main.layoutManager.panelBox.add(this.actor);
-        Main.ctrlAltTabManager.addGroup(this.actor, _("Top Bar"), 'emblem-system-symbolic',
+        Main.ctrlAltTabManager.addGroup(this.actor, _("Top Bar"), 'focus-top-bar-symbolic',
                                         { sortGroup: CtrlAltTab.SortGroup.TOP });
 
         Main.sessionMode.connect('updated', Lang.bind(this, this._updatePanel));