Now the story macro manages the story element sequence, while the
navigator macro listens for the navigation events, and modifies the
story tiddler as required. Also introduces a history tiddler that
retains the history stack so that we can animate navigation properly
(as distinct from animating the addition and removal of story elements).
Note that the zoomin storyview isn't quite finished, but this is a
stable point to commit these changes.
69a0c46447