Sets the 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_HORIZONTAL)
all have the same vertical requisition (GTK_SIZE_GROUP_VERTICAL),
or should all have the same requisition in both directions
(GTK_SIZE_GROUP_BOTH).
Sets the 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_HORIZONTAL) all have the same vertical requisition (GTK_SIZE_GROUP_VERTICAL), or should all have the same requisition in both directions (GTK_SIZE_GROUP_BOTH).