
Each plugin can define some informations about itself:
   * Its execution priority
   * Its prefered size
   * Its tip
   * Its title
   * Its CSS style
   * Its default ACLs for the admin/teacher/author/student roles.
   * Its container.
   * An evaluation function that may:
      * Define one part of the page.
      * Modify other page part.
      * Modify the student state or session.
      * Modify the statistics.
      * Generate final HTML from all the page part.
      * Implement authentication.

When an HTTP request come the plugins are called from the first to the last.
If the user do not have the access right to the plugin, it is not executed.

A question directory can have its own 'Plugins' directory in order
to have a specific display for the questionnary. NOT YET IMPLEMENTED






