Slot.notifyValueChanged purpose and behavior unclear

[burgerdev]

I don't quite understand why only output slots should listen on that signal, but even this does not work: Slot._sig_value_changed is not emitted anywhere.

[burgerdev]

Slot._sig_value_changed is emitted in OpArrayCache._ensureCached(), which is as far as I can tell only triggered upon requests to output slots Output and OutputHdf5.