Refactor event subscribers (remove Symfony Event dependency)

The event subscribers should have a more well-defined, descriptive and terse interface:

`TaskTrackerSubscriber` (or some other name)

```
`onStart(Tick $tick)`
`onFinish(Tick $tick)`
`onAbort(Tick $tick)`
`onProgress(Tick $tick)`
```

Use a more generic PubSub pattern implementation, and add a Symfony Event Dispatcher _client_ to maintain backwards compatibility.