void gtk_size_group_add_widget ( GtkSizeGroup* size_group, GtkWidget* widget )
Adds a widget to a
GtkSizeGroup. In the future, the requisition
of the widget will be determined as the maximum of its requisition
and the requisition of the other widgets in the size group.
Whether this applies horizontally, vertically, or in both directions
depends on the mode of the size group. See gtk_size_group_set_mode().
When the widget is destroyed or no longer referenced elsewhere, it will be removed from the size group.
The data is owned by the caller of the function.