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.
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.