oll-doc: remove mistakenly added file
oll-doc: minor syntax improvement
oll-doc: Cache HTML objs for display and file sep.

A Snippet() instance now has separate attributes for HTML
pages for display and file storage.
This increases memory usage but reduces HTML generation calls.
oll-doc: Factor out LibraryNavigation() class

This class generates HTML for a div with the navigation
structure of the library.