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

src/application-window.vala

@@ -3,11 +3,39 @@ namespace Example {
     [GtkTemplate (ui = "/org/gtk/exampleapp/window.ui")]
     public class ApplicationWindow : Gtk.ApplicationWindow {
 
+        [GtkChild]
+        private Gtk.Stack stack;
+
         public ApplicationWindow (Gtk.Application application) {
             GLib.Object (application: application);
         }
 
         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);
+            }
         }
     }
 }