Allow disabling New Folder button in File Choser

Bug #570516. Add a new property "create-folders" meaning that the
"New Folder" button should be displayed in the modes where it used
to be.

gtk/gtkfilechooserutils.c

@@ -117,6 +117,9 @@ _gtk_file_chooser_install_properties (GObjectClass *klass)
   g_object_class_override_property (klass,
 				    GTK_FILE_CHOOSER_PROP_DO_OVERWRITE_CONFIRMATION,
 				    "do-overwrite-confirmation");
+  g_object_class_override_property (klass,
+				    GTK_FILE_CHOOSER_PROP_CREATE_FOLDERS,
+				    "create-folders");
 }
 
 /**