Improve dirndl events

[nevella]

(Initial thoughts)

• Yes, use the GWT event pump:
  • this is more consistent with the general Alcina 'model process with objects, not recursion' (well kinda sorta - the gwt simple event pump is recursive. but does track event depth at least...)
  • more consistent with other (DOM, GWT-internal) events
• Factor dirndl event -> DOM event binding out to an 'eventsource' - which is null for ModelEvents. Use reflection to link (since really class-based, not instance) (/)
• ...? "don't implement the dispatch/TYPE mechanism" - yup, handlers are not used (rather DirectedLayout.Node ascent) - so possibly just have a single receiver (a singleton on directedlayout) which routes to the ascent algorithm actually, modified the gwt SimpleEventBus to allow dispatch with a preset handler
• Document:
  • An event lifecycle (dom - model - recast model)
  • Context uses (and Context)
  • propagation termination on handling