Inserts a new row before sibling.
If sibling is null, then the row will be appended to the end of the list. 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.
| this |
A `GtkListStore` |
| iter |
An unset `GtkTreeIter` to set to the new row |
| sibling |
A valid `GtkTreeIter` |