Display a big fat warning when switching to C
fix HTML markup
rerun po4a, no manual change
regenerate the translation files of the engine, too (no manual change)
Release 2.4 beta1

  Close to the release, but not there yet.

  * Improved translations:
    - Brazilian Portuguese: Now at 35% [Fred Maranhao]
  * User Interface:
    - Make it possible to copy/paste the secret session ID from the
      about dialog, so that the students can manually pass it to their
      teachers. Not as convenient as it ought to be, but it works.
    - Display a big fat warning when switching to C: that's still
      experimental, and not everything works for now. They're warned.
  * Pedagogical content:
    - welcome.array.maxvalue.Extrema: find min and max values
      [Valentin Giannini]
    - welcome.array.golomb.Golomb: compute the Nth of the surprising
      Golomb sequence [Valentin Giannini]
    - welcome.array.island.Island: small game with arrays of integers
    - sort.pancake.CohenPancake: Cohen's Algorithm for burned pancakes
    - lander.*: Integrate properly and port to any supported languages.

  * Still missing for the release:
    - French and Brasilian translations should be updated and completed
    - Switching identity is still broken (see #189)