* doc/footer.html: Make the footer XML compatible.
* doc/mainpage.dox: Fix references to modules.
* spot/tl/formula.hh: Introduce a hierarchy module.
* spot/tl/hierarchy.hh: Use it.
* doc/Doxyfile.in: Update to Doxygen 1.8.4
* doc/footer.html: Point to the mailing list.
* doc/mainpage.dox: Point to spot::translator,
and spot::kripke.
* src/ta/tgta.hh: Do not use \emph.
* src/tgba/succiter.hh: Fix rendering of example.
* src/tgba/tgba.hh: Correct documentation.
* src/tgbaalgos/cycles.hh: Improve rendering of
documentation.
* src/tgbaalgos/lbtt.hh, src/tgbaalgos/minimize.hh:
Document missing arguments.
