Method onReleased

src/fl/ui/button.h:100–104

    }

    int onReleased(function<void()> callback) FL_NOEXCEPT {
        int id = mReleaseCallbacks.add(callback);
        mListener.addToEngineEventsOnce();
        return id;
    }

    // Per-list removers. Each callback list maintains its own ID counter,
    // so an id returned by onChanged()/onPressed()/onReleased() is only