Fixed issue: MAGN4963

Issue: http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4963
    Check if the sub directory has name :"backup" or not
@Benglin Please review it
Few things:

1. Can we check ```if (directory.Name.Equals(```backup```))``` instead?
2. Why do we do ```WalkDirectoryTree``` then decide to not ```AddChildSampleFile```? Check early and continue early.
3. Rename ```subProperty``` to ```sampleFileEntry```, the old name doesn't make sense.
4. Use ```Configurations.BackupFolderName``` constant, don't hard code its value here (what if the name is changed later?)