py,st3m: Move WiFi and LED state handling out of Application class

Application class is the user's code, we shouldn't rely on it implementing system-level functionality as there are several ways for the user to accidentally break it.

This isn't a perfect place for it either, but it's at least better than before. It's going to be revisited in the future, perhaps by giving each app their own ViewManager instance that could then properly track application's lifecycle.

One functional difference is that st3m.wifi._onoff_wifi_update() is now being called when quitting any app, not just those that request a particular WiFi state in their config.

Also, call media.stop() when quitting an application.

Fixes #110 (closed)