void get_preferred_width ( GtkWidget* widget, gint* minimum_width, gint* natural_width )
Retrieves a widget’s initial minimum and natural width.
This call is specific to height-for-width requests.
The returned request will be modified by the
GtkWidgetClass::adjust_size_request virtual method and by any
GtkSizeGroups that have been applied. That is, the returned request
is the one that should be used for layout, not necessarily the one
returned by the widget itself.