managed but unmapped?

[Julian Bradfield <jcb+gtk dcs ed ac uk>]

I would like to set up the layout of my application assuming certain
widgets are present, but I don't want them visible and active.

In Xt, I would create them with MappedWhenManaged=False, realize
everything, and then map them when I wanted them visible.
I can't see how I do this in gtk: gtk_widget_hide seems to be the
equivalent of XtUnmanageChild as far as layout is concerned, and
gtk_widget_unmap can only be used after the widget's mapped.

How do I do this?