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

Release 2.4.8

• Engine:
  • Use appropriately the executionError field instead of
    compilationError to store entities' run time errors.
• Worlds:
  • ConsWorld: new subclass of BatWorld dedicated to recursive lists.
• Pedagogical content:
  • recursion.cons: new lesson on recursive lists (17 challenges)
  • #349, #355 : French typo
  • #356: write 'one', not '1' (was too similar to 'l') (fix #356)
  • #365: SelectionSort: don't sort the last cell, it's already done