py-st3m: feature: os_on_exit/enter

Applications that use custom on_enter/on_exit functions are required to include super().on_enter/exit() at this point in time. Forgetting about this is an easy way to break OS features at this point in time. Many standard applications from release did not do this right so we can assume some have copied the pattern. As the use of these more functions is getting more intricate this gets more pressing.

Proposition: Empty the Application.on_enter/exit bodies and move the contents to OS-side functions that get always called after/before so that a missing super().on_enter/exit() in application code cannot break OS features anymore. There's no harm in these functions being NOPs to begin with so that these extra lines can be omitted in all cases we think.