gtk_container_replace function.

[Chema Celorio <chema ximian com>]

I had to hack a function to replace a widget inside a container for
another widget, I am suggesting moving this functionality into gtk
because i have seen a couple of apps hack a similar functionality.
The prototype will be :

gtk_container_replace (GtkContainer *container, GtkWidget *old_widget,
GtkWidget *new_widget);

I have solved _my_ problem already, but i believe this functionality
will be better off inside gtk. When apps have to hack a function like
this they end up messing with the internals of the widgets/containers
and we should avoid it if possible.

regards,
Chema