Large commit, read it carefully, and make the required changes to
your theme:

- corrected some missing translations in story.module.  Oops!
- grealty simplified the "moderation threshold mechanism"(tm) so
  that module writers don't have to worry about this.  As a result
  story.module and book.module became a bit smaller and easier to
  grasp.
- greatly simplified new "category" and "topic" code which is soon
  going to replace the "section" code.  Needs more work though so
  hang on thight.
- includes/section.inc and modules/section.module are replaced by
  includes/structure.module and modules/structure.module.
- beautified example.theme a bit without adding HTML complexity:
  it is a good example but still useful as a theme
- made theme example use "categories" and "topics"
    --> TAKE A LOOK AT IT AND UPDATE YOUR THEME
- made theme marvin use "categories" and "topics"
    --> TAKE A LOOK AT IT AND UPDATE YOUR THEME
- added 2 new "story listings" to administrator interface of
  story.module to verify story integrity.
- optimized comment table a bit (work in progress)