Reset stop-propagation flag when triggering event

@macnibblet noticed that if an event listener on, say, the dispatch event called `stopPropagation` on the event, nothing was rendered in the response. On investigation, I discovered that the "stop propagation" flag on the `MvcEvent` was not being reset between different events that were being triggered. When triggering events in sequence, using the same event object, this is problematic, as the first listener in a subsequent trigger will be executed normally, but no others will be.

The solution is to reset the flag at the start of `trigger` (and `triggerUntil`). This PR does that, providing unit tests for the `EventManager`, and integration tests for the `Application` object.