lines of differing color are not equal
TurtleWorld.diffTo: i18n and improve error messages
recursion.tree: rework to make it easier to understand
retranslate everything
Release 2.2.6

  * SortingView: display the value on the left of temporal view, not
    the index
  * TurtleArt: force a newline every 5 exercises as it does not work
    well with Java7 otherwise.
  * TurtleArt.Flower: new exercise (provided by a student :)
  * GnomeSort: fix a bug in the correction, where we stay at i=0 when
    there is no previous pot instead of moving forward.
  * TurtleWorld.equals&diffTo: synchronized on shapes to avoid issues
    when the stop is pressed, for example
  * TurtleWorld: order the points in each segment so that the line
    (ab, cd) is detected as equal to (cd, ab)
  * TurtleWorld.diffTo: i18n and improve error messages
  * recursion.Circle: kill exercise. It is superseeded by turtleart.
  * recursion.Star: kill exercise. It is superseeded by turtleart.
  * recursion.tree: rework to make it easier to understand
    - each level is given its own color
    - we provide a subtree() that is correct (but the colors) to ease
      the debugging of user's code
  * As usual, fix a bunch of typos in the French translation