Move Strigi::DirLister to ArchiveReader::DirLister

The class Strigi::DirLister currently occurrs twice in Strigi.
This accidental circumstance is undesirable and hence the less important class is moved. Strigi::Dirlister in archivereader.h is not used outside of Strigi currently.

Therefore, while this change is not binary compatible, it poses not problems with current binaries using libstreams.
Strigi::DirLister is moved to Strigi::ArchiveReader::DirLister.
This makes sense since DirLister is a simple helper class for iterating through entries in a directory that ArchiveReader is reading.

A new Strigi release will follow in a couple of days unless problems are spottedwith this change.