100% Match of MxDSFile (#51)

* 100% Match of MxDSFile

* ...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.

* Add mxdssource.cpp

* mattkc feedback

* some accuracy improvements

* Use FOURCC macro

* Tirival solve in mxioinfo.cpp

* Update mxdsfile.cpp

0xFFFFFFFF -> -1

---------

Co-authored-by: Christian Semmler <mail@csemmler.com>

LEGO1/mxstring.h

@@ -16,6 +16,8 @@ public:
   void ToLowerCase();
   const MxString &operator=(MxString *);
 
+  inline const char *GetData() const { return m_data; }
+
 private:
   char *m_data;
   unsigned short m_length;