GtkSizeGroupMode of the size group. The mode of the size
group determines whether the widgets in the size group should
all have the same horizontal requisition (GTK_SIZE_GROUP_MODE_HORIZONTAL)
all have the same vertical requisition (GTK_SIZE_GROUP_MODE_VERTICAL),
or should all have the same requisition in both directions
(GTK_SIZE_GROUP_MODE_BOTH).gtkSizeGroupSetMode(object, mode)objectmode