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/slider.js

@@ -10,8 +10,8 @@ var SLIDER_SCROLL_STEP = 0.02; /* Slider scrolling step in % */
 var Slider = GObject.registerClass({
     Signals: {
         'drag-begin': {},
-        'drag-end': {}
-    }
+        'drag-end': {},
+    },
 }, class Slider extends BarLevel.BarLevel {
     _init(value) {
         super._init({