Show feedback dialogs and validation errors from JSON responses
Refs #274. Sending full URL to client framework for notification dialog action-forms.
Refs #274. New notification messages interface.
- Pass an implementor of
signalEventFinally()instead of a callback routine.
- Implemented that interface in Standard module, with an overridable procedure that returns the action list of the notification dialog
- Defined a dialog "transient" when none of its action has any data (so it's rendered as a link instead of a FORM with buttons)