Prepends a new row to this.
If parent is non-null, then it will prepend the new row before the first child of
parent, otherwise it will prepend a row to the top level. 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 prepended row |
| parent |
A valid TreeIter, or null |