gui: Unselect data in data editor when the datasheet is replaced.

The active cell in the PsppireSheet remembers its contents even if
the whole sheet gets replaced, so we have to unselect the cell when
this happens to avoid replacing the cell's contents by whatever was
there before.

Bug #30502.
Thanks to Michel Boaventura for reporting the problem and John
Darrington for diagnosing it.