default python apps: use super.on_enter/on_exit reliably

Considering things like !243 (merged) rely on Application's on_enter and on_exit, it's best if we set in default apps the example of using super().on_enter/on_exit() in relevant functions. It's not a significant overhead.

@moon2: Several of your projects had avoided it as it worked fine on shoegaze, I'm adding it to them regardless as I think you're especially interested in something !243 (merged)-shaped.