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).