Added is_creating parameter for BaseModelView.on_model_change

On my `User` model I have two special required fileds `password` (hash digets) and `password_salt` (random string) which genereted from user readable password (`password = hash(readable_password + password_salt)`). When user create account my code populate it.

So now I want create users with `flask-admin`:  populate fileds when `on_model_change` will be called.

When I override `on_model_change` with my code then on creating fields populated but on update it also will be updated. I prefere avoid this fileds updating on update.