try to ensure that the lecture chooser is not bigger than the window by setting its ancestor to MainFrame
sort.insertion: fix path to img after recent package reorg
all the needed html texts for the dutchflag problem (not translated yet)
recursion.koch: give one color per side to ease the understanding
recursion.koch: retranslate to french
sort.CocktailSort2: don't use ++ but += 1 in pseudocode
Release 2.2.7

  * recursion.spiralUse: rewrite to leverage the creative mode,
    so that students don't spend too much time seeking for the reward.
  * recursion.tree: make the color changing robust to negative steps
  * recursion.tree: forbid teleporting
    This was used by some students to make the method idempotent.
  * recursion.sierpinski: rewrite to simplify the code to be written
  * recursion.hanoi: Simplify by explicitely passing the index of the
    unused peg to the recursive function.
  * TurtleWorld: synchronize to avoid a concurent modification when a
    shape is added while we are computing the toString() of the world
  * sort.dutchflag: New universe/exercise on the Dutch Flag Sorting
  * sort.pancake: Correctly reset the move count on need
  * sort.insertion: fix path to img after recent package reorg
  * recursion.koch: give one color per side to ease the understanding
  * sort.CocktailSort2: don't use ++ but += 1 in pseudocode