ISIS-478: working towards entity choices (not there yet)

* refactored EntityLinkSelect2Panel so that the setChoices can be easily called for argsIfAvailable
* ReferencePanel overrides addFormComponentBehaviour and updateChoices, delegating to entityLink in both cases.
ISIS-500: making EntityIconAndTitlePanel easier to subclass

in addition:
* removed some unused ComponentFactory's
* added in example css in a ToDo app's application.css to demonstrate how to hide the icon
* make some demo code in QuickStartApplication easier to enable
* renamed PageRegistrySpi -> PageClassRegistrySpi
* added example override of PageClassList