Sets the row of the model that is currently displayed by the `GtkCellView`.
If the path is unset, then the contents of the cellview “stick” at their last value; this is not normally a desired result, but may be a needed intermediate state if say, the model for the `GtkCellView` becomes temporarily empty.
| this |
a `GtkCellView` |
| path |
a `GtkTreePath` or null to unset. |