Provide an icon to the window.
Définit une icône à une fenêtre.
_window_ : ObjGtk : any window already created.
_filename_ : P : any image file supported by GTK+ into the active Scol partition. If _filename_ is set to 'nil', default icon is allowed.
If the image is animated, the window's icon will not be animated.
Return : ObjGtk : the same window or nil if error (see message in the log).
Note to dev. : TODO : remove scol icon if _filename_ is nil (no icon allowed) ...?