Allow using wxZlib{Input,Output}Stream too, meaning that the sample can
now also work with .gz files, handling them as a degenerate (because
containing only a single file) special case of archives.
The changes here -- which should be viewed ignoring whitespace to be
actually readable -- don't modify the existing code and just add the
possibility to use a wxFilterClassFactory if there is no
wxArchiveClassFactory corresponding to the specified file extension.
6da3c3c34d