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/ctrlAltTab.js

@@ -27,8 +27,8 @@ const CtrlAltTabManager = new Lang.Class({
     _init: function() {
         this._items = [];
         this.addGroup(global.window_group, _("Windows"),
-                      'emblem-documents-symbolic', { sortGroup: SortGroup.TOP,
-                                                     focusCallback: Lang.bind(this, this._focusWindows) });
+                      'focus-windows-symbolic', { sortGroup: SortGroup.TOP,
+                                                  focusCallback: Lang.bind(this, this._focusWindows) });
     },
 
     addGroup: function(root, name, icon, params) {