make this work again. =)

* removed the updated signals from IonInterface; we already have that via DataEngine::updated
* respond to source add/removed signals from the Ions (which are now DataEngines themselves)
* call setData in the WeatherEngine when the ion calls updated

essentially, we're chaining engines. drawbacks right now include:

- the sources between all ions must be uniquely named. this means that right now you can't have "Calgary, AB" from two different weather sources. not sure that matters
- with this method we have 2 copies of the data in memory (one in the ion, one in the weather engine) and cycle through the data in WeatherEngine; not the greatest. not sure this matters either, but it is a place for possible future optimization