Query tool execute to file prompting

Hi!

This patch attempts to address the issue that at least I've heard a
couple of time, of that the "execute to file" feature doesn't prompt
for the filename until after it's finished. This is quite annoying
when running multi-hour queries.

Other than a review please, I couldn't find where we actually destroy
the QueryExecInfo object allocated in frmQuery::execQuery(). It gets
picked up in OnQueryComplete(), but I don't see a delete anywhere. Is
it automatically deleted by the wx stuff (in which case I have a leak
of qi->toFileExportForm that I have to take care of), or do we have a
leak there today (in which case I have that leak to take care of :D).

--
Magnus Hagander
Me: http://www.hagander.net/
Work: http://www.redpill-linpro.com/