Update RinWPFAPP/Models/Model.cs

1. add logic comparing not only directories, but also files.
2. add error handling that directory/file has been deleted.
3. fix a bug(the repetition of get sub directories).
4. improve a way of displaying path (absolute path -> relative path).
5. improve performance: sort before comparing. if sourceFilePath < destFilePath, then break.
   (compare not only MD5, but also filePath)