Set or get the window transparency.
Définit la valeur de transparence d'une fenêtre ou retourne la valeur courante.
_window_ : ObjGtk : any window already created.
_trans_ : I : the value of the window transparency. The range is between 0 and 255 : 0 is fully transparency, 255 is fully opaque (by default). To get the current value, set this parameter to nil.
Return : I : _trans_ or the current value if _trans_ is nil.