cleanup: Port GObject classes to ES6 classes

GJS added API for defining GObject classes with ES6 class syntax
last cycle, use it to port the remaining Lang.Class classes to
the new syntax.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell-extensions/issues/30

extensions/example/prefs.js

@@ -16,13 +16,10 @@ function init() {
     Convenience.initTranslations();
 }
 
-const ExamplePrefsWidget = new GObject.Class({
-    Name: 'Example.Prefs.Widget',
-    GTypeName: 'ExamplePrefsWidget',
-    Extends: Gtk.Grid,
-
+const ExamplePrefsWidget = GObject.registerClass(
+class ExamplePrefsWidget extends Gtk.Grid {
     _init(params) {
-        this.parent(params);
+        super._init(params);
         this.margin = 12;
         this.row_spacing = this.column_spacing = 6;
         this.set_orientation(Gtk.Orientation.VERTICAL);