Appends a new row to this.
If parent is non-null, then it will append the new row after the last child of
parent, otherwise it will append 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 appended row |
| parent |
A valid TreeIter, or null |