msvc: skip copying Data dir and precompiled dlls to Binary dir if they are already there (unless the source files are newer).

Also fix a bug which was preventing DolphinWX/AfterBuild target from being completely skipped.
Merge pull request #848 from shuffle2/msvc-improvements

msvc: skip copying Data dir and precompiled dlls to Binary dir if they are already there (unless the source files are newer).