cleanup: Require "dangling" commas

Since ES5, trailing commas in arrays and object literals are valid.
We generally haven't used them so far, but they are actually a good
idea, as they make additions and removals in diffs much cleaner.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/805

js/ui/panelMenu.js

@@ -203,7 +203,7 @@ class SystemIndicator extends St.BoxLayout {
         super._init({
             style_class: 'panel-status-indicators-box',
             reactive: true,
-            visible: false
+            visible: false,
         });
         this.menu = new PopupMenu.PopupMenuSection();
     }