Major reworking of filesystem traversal, including history URLs.

* Removed Datastore.Events.Breadcrumbs
* Replaced with Datastore.Events.Traversal that supports a single event
  "navigate" that navigates to a given path
* Old views--directory listings of the ancestors--are now removed from
  the DOM and rebuilt if necessary
* Breadcrumb view is now much more rebust--updating it no longer relies
  on past state
* History URLs now fully supported--root-relative paths will correctly
  bring up the view of the corresponding directory or file