Sets whether child should appear in a secondary group of children.
A typical use of a secondary child is the help button in a dialog.
This group appears after the other children if the style is gtk_buttonbox_start, gtk_buttonbox_spread or gtk_buttonbox_edge, and before the other children if the style is gtk_buttonbox_end. For horizontal button boxes, the definition of before/after depends on direction of the widget (see set_direction). If the style is gtk_buttonbox_start or gtk_buttonbox_end, then the secondary children are aligned at the other end of the button box from the main children. For the other styles, they appear immediately next to the main children.
| this | |
| child |
a child of this |
| is_secondary |
if true, the |