status: Refine system status menu

* switch to a one-column layout (and adjust strings/widths
   accordingly
 * remove separator before system menu
 * add link to account settings to user submenu for consistency

https://bugzilla.gnome.org/show_bug.cgi?id=751377

js/ui/panel.js

@@ -697,7 +697,6 @@ const AggregateMenu = new Lang.Class({
         this.menu.addMenuItem(this._location.menu);
         this.menu.addMenuItem(this._rfkill.menu);
         this.menu.addMenuItem(this._power.menu);
-        this.menu.addMenuItem(new PopupMenu.PopupSeparatorMenuItem());
         this.menu.addMenuItem(this._system.menu);
     },
 });