• added Path::getFilename()
• added Path::getFilenameWithoutExtension()
• added Path::getExtension()
• added Path::hasExtension()
• added Path::changeExtension()
• Path::makeRelative() now works when the absolute path and the base path
  have equal directory names beneath different base directories
  (e.g. "/webmozart/css/style.css" relative to "/puli/css")