Asks to keep a window at the top (foreground). Note that operating system or the user can force the position of the window.
Demande à garder une fenêtre en avant-plan (devant les autres fenêtres).
_window_ : ObjGtk : any window already created.
_value_ : I : The window stays on the top if _value_ = 1. Another value unset it..
Return : ObjGtk : the same window or nil if error.