ISIS-800: improvements to the wizard

... sorted out that issue with drop-down choices not being disabled (was a bug in ValueSelect2ChoicesPanel, not in EntityPropertiesForm itself)

Also:
- extended the ToDoItemWizard to demonstrate
- refactored/improved/better documented the Wicket viewer's use of the Wicket event bus (even though it turns out that I didn't need to use it in order to fix the issue...).