Update callers

Adapt all our tests and examples to the new initialization api.

tests/syncscroll.c

@@ -20,7 +20,7 @@ main (int argc, char *argv[])
   GtkWidget *win, *box, *tv, *sw, *sb;
   GtkAdjustment *adj;
 
-  gtk_init (NULL, NULL);
+  gtk_init ();
 
   win = gtk_window_new (GTK_WINDOW_TOPLEVEL);
   gtk_window_set_default_size (GTK_WINDOW (win), 640, 480);