cleanup: Use method syntax

Modern javascript has a short-hand for function properties, embrace
it for better readability and to prepare for porting to ES6 classes.

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

extensions/example/prefs.js

@@ -21,7 +21,7 @@ const ExamplePrefsWidget = new GObject.Class({
     GTypeName: 'ExamplePrefsWidget',
     Extends: Gtk.Grid,
 
-    _init: function(params) {
+    _init(params) {
         this.parent(params);
         this.margin = 12;
         this.row_spacing = this.column_spacing = 6;