insert_after
Description:
Inserts a new row after sibling.
 If sibling is null, then the row will be prepended to parent ’s children. If 
      parent and sibling are null, then the row will be prepended to the toplevel. If both
       sibling and parent are set, then parent must be the parent of sibling. When sibling
       is set, parent is optional.
iter will be changed to point to this new row. The row will be empty after this function is called. To fill in values, you need to
       call @set or 
        set_value.
Parameters:
| this | |
| iter | 
           An unset TreeIter to set to the new row  | 
      
| parent | 
           A valid TreeIter, or null  | 
      
| sibling | 
           A valid TreeIter, or null  |