public void set_opacity (double opacity)
Warning: set_opacity is deprecated since "3.8".
Request the windowing system to make this partially transparent, with opacity 0 being fully transparent and 1 fully opaque.
Use gtk_widget_set_opacity instead.
(Values of the opacity parameter are clamped to the [0,1] range.) On X11 this has any effect only on X screens with a compositing manager running. See is_composited. On Windows it should work always.
Note that setting a window’s opacity after the window has been shown causes it to flicker once on Windows.
desired opacity, between 0 and 1