Stop using gtk_main and gtk_main_quit

Replace these calls with direct use of GMainContext api.

tests/syncscroll.c

@@ -63,7 +63,8 @@ main (int argc, char *argv[])
 
   gtk_widget_show (win);
 
-  gtk_main ();
+  while (TRUE)
+    g_main_context_iteration (NULL, TRUE);
 
   return 0;
 }