Add a simple Javascript example

This shows how to use a layout manager in a widget,
implemented in javascript. The example sets up the
environment for running from the toplevel dir, assuming
that the build dir is called 'build'.

examples/labels.js

@@ -0,0 +1,50 @@
+#!/usr/bin/env -S GI_TYPELIB_PATH=${PWD}/build/gtk:${GI_TYPELIB_PATH} LD_PRELOAD=${LD_PRELOAD}:${PWD}/build/gtk/libgtk-4.so gjs
+
+imports.gi.versions['Gtk'] = '4.0';
+
+const GObject = imports.gi.GObject;
+const Gtk = imports.gi.Gtk;
+
+const DemoWidget = GObject.registerClass({
+  GTypeName: 'DemoWidget',
+  }, class DemoWidget extends Gtk.Widget {
+
+  _init(params = {}) {
+       super._init(params);
+
+       let layout_manager = new Gtk.GridLayout ();
+       this.set_layout_manager (layout_manager);
+       this.label1 = new Gtk.Label({ label: "Red",
+                                     hexpand: true,
+                                     vexpand: true });
+       this.label1.set_parent (this);
+       let child1 = layout_manager.get_layout_child (this.label1);
+       child1.set_row (0);
+       child1.set_column (0);
+
+       this.label2 = new Gtk.Label({ label: "Green",
+                                     hexpand: true,
+                                     vexpand: true });
+       this.label2.set_parent (this);
+       let child2 = layout_manager.get_layout_child (this.label2);
+       child2.set_row (0);
+       child2.set_column (1);
+    }
+});
+
+// Create a new application
+let app = new Gtk.Application({ application_id: 'org.gtk.exampleapp' });
+
+// When the application is launched…
+app.connect('activate', () => {
+    // … create a new window …
+    let win = new Gtk.ApplicationWindow({ application: app });
+    // … with a button in it …
+    let widget = new DemoWidget();
+    win.set_child(widget);
+    win.present();
+});
+
+// Run the application
+app.run([]);
+