Accessor of the slot opacity of the gtk-widget class.
The generic function gtk-widget-opacity fetches the requested opacity for this widget.
The generic function (setf gtk-widget-opacity) request the widget to be rendered partially transparent, with opacity 0 being fully transparent and 1 fully opaque.
Opacity values are clamped to the [0,1] range. This works on both toplevel widget, and child widgets, although there are some limitations: