Dot decorate a folder name.

def _foldername(self, additionalpath=""):
        """Dot decorate a folder name."""
        if not self._foldername_cache.get(additionalpath):
            fn = joinpath(self.base, self.folder, additionalpath) \
                if not self.is_subfolder \
                else joinpath(self.base, ".%s" % self.folder, additionalpath)
            self._foldername_cache[additionalpath] = fn
        return self._foldername_cache[additionalpath]