QQuickTextNode must be part of the Qt public API

As suggested in QTBUG-31683, QQuickTextNode must be part of the Qt public API and documented.

A public API would permit to user to develop their custom items without the need to hack the private Qt API or rely on QPainter API.

Moreover this component is quite important since the handling of glyph and text layout is not trivial in OpenGL and require a lot of codes which is already implemented in Qt. In comparison the painting of an antialiased  curve is far more simple to achieve.

At least it would be nice to explain why it was not done : lack of time, code was not designed to be public, easier to make crappy code private ...