Inserts a new row after sibling.
If sibling is null, then the row will be prepended to the beginning 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 | |
| iter |
An unset TreeIter to set to the new row |
| sibling |
A valid TreeIter, or null |