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

- The code used System.getProperty("file.separator") for detection of unix/windows directory separator. This technique may be dangerous now, because the SLOCCount report (output from sloccount utility) may be generated on slave (unix/windows), the same file then transfered to master (windows/unix) and lazy loaded/parsed on master. So both unix and windows separators should be considered during the parsing.
- Directory separator member variables removed from SloccountReport, System.getProperty("file.separator") is no longer used, because it lost its sense and may return not suitable value.
- Method extractFolder() tests unix separator first and then the windows one. If there is no match, empty string will be returned. -1/not found returned from lastIndexOf() will never cause StringIndexOutOfBoundsException.
- Method updateRootFolderPath() updated to try unix separator first and then the windows one.
- Unit/integration tests for the changes above created.
- Tested only on Linux, I don't have Windows machine available.
- This commit is partially inspired by André Wegmüller's two years old pull request. The code is now too incompatible, sorry.
- If no file is matching the input pattern, the publisher will output a warning to the build log.