app :: AppComponent :: notify_observers
event to all observers by calling AppObserver::on_event
	# Propagate `event` to all `observers` by calling `AppObserver::on_event`
	fun notify_observers(event: AppEvent)
	do
		for observer in observers do
			observer.on_event(event)
		end
	end
					lib/app/ui.nit:118,2--124,4