ListenerHandle should have been an alias for uint64_t

[KyrietS]

I don't exactly know why I decided to make ListenerHandler as a custom struct. It doesn't make sense and I think it should be an uint64_t for the sake of simplicity.

(maybe some time ago I probably wanted a RAII "token" that would remove the listener automatically)

[KyrietS]

Now I remember. I wanted to have strong-immutable-type for a handler. Using uint64_t would mean that it is just a number without any meaning. Not sure what would be better for a user, have a restrictive ListenerHandler that won't let anyone destroy the value or a simple number and let people handle it however they want...