Use gtk_window_set_child throughout

Replace all uses of gtk_container_add on windows
by gtk_window_set_child.

tests/testentrycompletion.c

@@ -314,7 +314,7 @@ main (int argc, char *argv[])
   g_signal_connect (window, "destroy", G_CALLBACK (quit_cb), &done);
   
   vbox = gtk_box_new (GTK_ORIENTATION_VERTICAL, 2);
-  gtk_container_add (GTK_CONTAINER (window), vbox);
+  gtk_window_set_child (GTK_WINDOW (window), vbox);
     
   label = gtk_label_new (NULL);