* ...almost, MxDSFile::Open is still not quite matching but all of the
other methods are 100% matching.
* Turns out that most of the virtual methods and some of the members are
actually on the MxDSSource base class, which I've pulled out as part
of this.
* In order to implement the methods I added the MXIOINFO class, which
seems to be a thin wrapper around the MMIOINFO windows structure. We
can tell this because MMIOINFO::~MMIOINFO was included in the DLL
exports, and calls down to a function which deconstructs something
looking exactly like MMIOINFO.
