The main part consists of an interface: ISfQt_canvas, and a concrete class,
SfQt_canvas.
The later uses function objects as its on_init and on_update functions,
so the entire behavior is set-up by client code.
An alternative would be to derive from ISfQt_canvas interface, overriding
the virtual methods directly.
The interface code is heavily inspired by this article: http://becomingindiedev.blogspot.com/2013/10/qt-5-and-sfml-20-integration.html.
If you know a better, cleaner, simpler or more idiomatic way to mix SFML and Qt, please let me know.