application-window: Add stack child, and implement open() method.

    [GtkTemplate (ui = "/org/gtk/exampleapp/window.ui")]

    [GtkTemplate (ui = "/org/gtk/exampleapp/window.ui")]

    public class ApplicationWindow : Gtk.ApplicationWindow {

    public class ApplicationWindow : Gtk.ApplicationWindow {

        [GtkChild]

        private Gtk.Stack stack;

        public ApplicationWindow (Gtk.Application application) {

        public ApplicationWindow (Gtk.Application application) {

            GLib.Object (application: application);

            GLib.Object (application: application);

        }

        }

        public void open (GLib.File file) {

        public void open (GLib.File file) {

            var basename = file.get_basename ();

            var scrolled = new Gtk.ScrolledWindow (null, null);

            scrolled.show ();

            scrolled.hexpand = true;

            scrolled.vexpand = true;

            var view = new Gtk.TextView ();

            view.editable = false;

            view.cursor_visible = false;

            view.show ();

            scrolled.add (view);

            stack.add_titled (scrolled, basename, basename);

            try {

                uint8[] contents;

                if (file.load_contents (null, out contents, null)) {

                    var buffer = view.get_buffer ();

                    buffer.set_text ((string)contents);

                }

            } catch (GLib.Error e) {

                GLib.warning ("There was an error loading '%s': %s",

                              basename, e.message);

            }

        }

        }

    }

    }

}

}