[Vala] Set a RGBA colourmap on Gtk.Window

[David Gomes <davidrafagomes gmail com>]

Hey, I read the following on Valadoc here:
http://valadoc.org/vte-2.90/Vte.Terminal.background_transparent.html,

Note: When using a compositing window manager, you should instead set a
RGBA colourmap on the toplevel window, so you get real transparency.

Now my doubt is how to set an RGBA colormap on a widget or on a Gtk.Window?

Thanks.

-- 
David Gomes