Converts child_path to a path relative to this.
That is, child_path points to a path in the child model. The rerturned path will point to the same row in the filtered
model. If child_path isn’t a valid path on the child model or points to a row which is not visible in
this, then null is returned.
| this |
A `GtkTreeModelFilter` |
| child_path |
A `GtkTreePath` to convert. |
|
A newly allocated `GtkTreePath` |