[JENKINS-21419] Can't open sloccount table portlet

- Links to specific folders in folder details don't work.
- All Windows backslashes are now replaced by Unix slashes at the very beginning. Only unix slashes are considered in the rest of the code, which significantly simplifies it and prevents possible errors.
- SloccountReport.extractFolder() is now used in FolderFileFilter.include() to fix the same issue that was present in extractFolder().
- Constant DIRECTORY_SEPARATOR constant is used instead of hardcoded "/".