tree/actor_common: allow custom handler for `a_bye` message in event listener
tree/actor_common: allow custom handler for `a_bye` message in event listener

In prev impl `a_bye` message was overloaded by `ev_listener_actor` for
cleanup purposes. Allow custom event listener character to also overload
this message.

Current approach is to create custom `a_bye` message and propagate it
into a copy of character. However holding a character copy shouldn't be
expensive (callback is anyway stored only once).

uentity/blue-sky-re,uentity/blue-sky-re,uentity/blue-sky-re