[Kst] Strange problem using the change data file tool

[Brisset, Nicolas]

> > 3) we find a way to reduce the number of calls to datasource 
> > constructors and 2) becomes acceptable (but this will 
> surely take more
> > time)
> 
>    May not happen due to the desire to auto-detect files.
Why is that so ? If a datasource provides efficient understands...() and
fieldListFor...() methods, I believe auto-detection can live without
needing to call the constructor. And if it does call it, then for those
sources that seem to be able to read the file, the instance should be
kept.

Apart from that point, I also had a bug in the way _valid was set, which
is why d2asc did not work. I am actually still wondering why kst itself
does not use _valid to keep track of the status of in-use datasources.
Currently, even if a datasource sets _valid = false, kst will still try
to update it and read data. 'Sounds weird... Well, this is probably not
the right time to discuss this but we might come up with ideas how to
make a smart use of that property.

Nicolas