StoryIncludes passage list updated on startup

* Renamed most all codebase instances of "external", used to refer to
passages in included files, to "included".
* The internal included passage list, used for link highlighting, etc.,
for each story is now assembled whenever the story is opened, rather
than remaining empty until a build.
* TweeLexer's external link checker now requires all relative-path URIs
to begin with "/". This is a concession that it's altogether too hard to
distinguish relative URIs solely by the placement of a period.
* Darkened the link connector arrows a tad, again.