* Keep retained tiddler subdirectories on save
Make generateTiddlerFilepath keep a retained tiddler's recorded subdirectory
and produce a valid filename for every path segment.
* Normalise originalpath separators to "/" before the sanitiser, so recorded
subdirectories survive
* Validate each path segment (reserved names, trailing dots or spaces), so
"sub/CON" is valid on every OS
* Run that pass after transliteration, which can itself create a reserved name
* Add filesystem and adversarial specs, covering generateTiddlerFileInfo and
generateTiddlerExtension
* update changenote
