Initial checkin of MS Cabinet file format archive reader

This class subclasses KArchive, and will (in the future :-) allow access to file that are archived in the "Cabinet" file format.

That format is documented, and there are other readers (e.g. cabextract) that handle it.

This still has a long way to go. I'm still struggling with the headers, and haven't yet done any of the decompression stuff.