Replace `NotificationCallback` with `cxx::function`

## Brief feature description

Since a heap-free and exception-free `cxx::function` is now available use them, instead of `NotificationCallback` in `popo::WaitSet` and `popo::Listener`.

## Detailed information

* With this change, one will be able to store arbitrary callables inside `WaitSet` and `Listener`. Currently only static methods or free functions are allowed.

Start by locating the popo::WaitSet and popo::Listener definitions and their uses of NotificationCallback. Check how cxx::function is already used, then update both APIs so arbitrary callables can be stored instead of only static methods or free functions; done means both components use cxx::function consistently.

Written by the indexing model from the issue text.