how to set gtk widget translucent

["Zhu, Jack" <jack zhu intel com>]

Hi, All,
I want to add a gtk widget on top of another gtk widget, then set Alpha
value to the widget above, make it translucent, for example, I want to
add a translucent seekbar on a window
1. use gdk_window_set_opacity?
2. use gdk.window.cario_create?
3. only gtk+-2.12 support this new feature? 
4. Compiz can do that work, but seems, just put one translucent window
on top of another, it can not handle any kind of gtk widget, is that
true?
Looking forwards to anyone's help.