(Code: --: todo, ++: done)

-- Code beautification. (permamnent)

-- Document with "doxygen" the code. (permamnent)

-- Docment the program better, especially how to write the data files.

++ Simplify the input files by automaticaly detecting newly added lesson 
   sections.

++ Load/Reload buttons to permit changing the definition file without
   restarting the application.

-- Clean the memory allocated for the VDKFileTreeDialog after the window
   is closed. Now the memory is freed by the garbage collector at program
   termination.

-- The Reload should not unselect the current selected lessons, but preserve 
   the current selections. The Load and Reload should be changed not to 
   start a new quizz set, eventually.
