UnityContent: support multiple event handlers

[stefanb2]

• remove IUnityEvent
• add UnityEvents class
  • AddEventListener() adds a callback for an event name
  • DispatchEvent() calls all callbacks for an event name
  • errors from event handlers are ignored
  • the first event handler returning truthy aborts dispatch
• UnityContent
  • add UnityEvents class property for global events
  • only install one callback per global event name
  • add UnityEvents instance property for instance events
• some other minor fixes I noticed while developing the code

Fixes #109