Don't pass `DIFFITEM *` by casting to `uintptr_t` (1)

Don't pass `DIFFITEM *` by casting to `uintptr_t` (1)

* Change DIFFITEM from `struct` to `class`
* Incorporate `ListEntry` class directly into DIFFITEM
* Change handling of m_Root (now m_pRoot) and `children`
* Eliminate all casting between `DIFFITEM *` and `uintptr_t`
* Isolate all manipulation of `DIFFITEM` linkage to be within the DIFFITEM class.

* Add `#include "StdAfx.h"` and `#define new DEBUG_NEW` to a few files
* Cleanup various related comments
* Eliminate the `ListEntry.h` file.  Also, it was never part of the project's Solution Explorer
tree.