[Kst] [Bug 111570] Change Data File's 'All From" should only include files that are currently in use.

[Barth Netterfield]

Perhaps we should reconsider this... some data sources (eg, ascii) can keep a 
fair amount of data around....  Maybe a 'purge files' type option?

> The reason the rest are listed is because they are cached (a sort of
> fly-weight pattern implementation).  We could remove the unused files from
> the combobox but I still want to keep them around in memory for performance
> reasons.