[gtk+] gtk-demo: Fix the assistant demo

[Matthias Clasen <matthiasc src gnome org>]

commit ad9c70537dfd65e49d5e64b8c100fb7d74eea347
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Oct 6 19:38:16 2017 -0400

    gtk-demo: Fix the assistant demo
    
    The progress bar was no longer expanding as intended,
    due to the demise of GtkBox::expand.

 demos/gtk-demo/assistant.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/demos/gtk-demo/assistant.c b/demos/gtk-demo/assistant.c
index a1c0837..c39d0b2 100644
--- a/demos/gtk-demo/assistant.c
+++ b/demos/gtk-demo/assistant.c
@@ -144,8 +144,11 @@ static void
 create_page4 (GtkWidget *assistant)
 {
   progress_bar = gtk_progress_bar_new ();
-  gtk_widget_set_halign (progress_bar, GTK_ALIGN_CENTER);
+  gtk_widget_set_halign (progress_bar, GTK_ALIGN_FILL);
   gtk_widget_set_valign (progress_bar, GTK_ALIGN_CENTER);
+  gtk_widget_set_hexpand (progress_bar, TRUE);
+  gtk_widget_set_margin_start (progress_bar, 40);
+  gtk_widget_set_margin_end (progress_bar, 40);
 
   gtk_widget_show (progress_bar);
   gtk_assistant_append_page (GTK_ASSISTANT (assistant), progress_bar);