Modified the section labels to have a little 'node picture'. This represents a day
or so of experimentation, but I'm not convinced that it is optimal, so don't mind
if we don't end up using it :) It's quite fun- it writes counters representing
the total number of sections to the .aux file, and then reads them back (so it takes
two compiles to get correct if there are any changes). Even if we don't end up
using it, the coding was fun- perhaps it'll lead us to something.