Add tests to verify if the exercise's solution passes and a wrong code not
don't compute the thread stack if there is no listener needing it
avoid a synchronized if there is no listener to inform
remove the biggest recursive figures: the induced load is too large

I'm still surprised to see that we are creating 51M lines with the
three commented worlds, resulting in 3+Gb memory. I can't remember
changing anything about the lines that would explain that student load
surge.
fix TurtleWorld.equals()
recursion.spiralUse: add another nice example (fix #366)
Lesson constructor waits for all init threads' completion
refresh translations (no manual change)
Release 2.4.9

  Recursion lesson is usable again, sorry for the inconvenience.
  (we really need more tests, grmbl)

  * Engine:
    - Lesson constructor waits for all init threads' completion
  * Worlds:
    - Turtle: equal() was too permissive after recent work on merging
      lines that were too picky. Now take the world name into account
      in equality, so that we can switch from an empty world to
      another empty world in the UI.
  * Pedagogical content:
    - recursion: remove the largest fractals that were too heavy
    - #366: recursion.spiralUse: add another nice picture
Merge branch 'master' into add-tests-exercisetemplated
Debug SimpleExercise's scala entity