Improve interrelation with GNU diffutils source

Improve interrelation with GNU diffutils source

- Remove four *.c files never used by diffutils or WinMerge

- Change `Src/CompareEngines/DiffUtils{.c|.h}` to be
`./Wrap_DiffUtils{.c|.h}` to remove confusion about the **diffutils**
name.  These files are part of WinMerge, not GNU diffutils.

- Change Solution Explorer to have the entry **GNU diffutils** rather
than **DiffEngine**

- Copy solution changes from VS2017 to VS2015