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Release 2.4.7

  Python is usable again, sorry for the inconvenience.

  * Engine:
    - Don't even compare the worlds when there was a compilation error
      Prevents python code from failing after reaching the solution.
  * Worlds:
    - Turtle: Merge lines that are in line with each other before
      comparing equality. You can now walk half of a segment, the half
      will be removed. You can walk to your destination in large
      amount of very small lines that are all depicting a long line,
      parts will be merged together. I hope that now, that damn
      world equality detection will not return false negative anymore!
      This fixes #338 and #339 (which are not exact duplicates).
    - Turtle: Show the superflous and missing shapes when the amount
      of shapes does not match.
    - #344 Turtle: Display the coordinates of the mouse.
      That helps reading the differences between worlds, if any.
    - #309 Bat: Edit the python template in order to test correctly
      the exercise's winning conditions instead of passing it.
  * Pedagogical content:
    - #341: French typo
    - TurtleArt: rename challenges from Bill Kerr to credit the author.
      http://billkerr2.blogspot.fr/2009/08/40-maths-shapes-challenges.html