[8u] API Request: RT-25613, ObservableValue should have a hasListener(listener) method

[Tom Schindl]

[...]

> Actually even when you would rely on the order, in situations when you
> call ensureListener, you don't really know if the listener is already
> there. So you might really expect that listener would be added at this
> point as the last. It's just that will always be the outcome if the
> default implementation is used.

It would be even worse if this would be different in none default
implementations.

Tom