Merge: FlatText is a private class

This PR does two things:
* Define `FlatText` as a private class. Since every clients were using an intrude import, their code do not change.
* Remove nullable warning in text::flat

Pull-Request: #1752
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>