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).