* Exercise Viewer - Main Panel: added initial FXML layout file.
* Exercise Viewer - Main Panel: work on FXML layout file.
* TODO updates.
* Exercise Viewer - Main Panel:
  * completed FXML layout definition.
  * added initial controller implementation.
* Exercise Viewer - Main Panel: completed controller implementation.
* Exercise Viewer - Main Panel: Layout fix.
* Exercise Viewer - Optional Panel: added initial FXML layout file.
* Exercise Viewer - Optional Panel: added controller implementation.
* Updated TODOs (completed tests).
* Exercise Viewer - Main and Optional Panel: layout fixes.