For unknown reasons, GVC mixer control can sometimes signal a new
device with the same id as one that was added before. This means that
in `device_added_cb`, a duplicate entry with that id is created, in my
case with a different name. However, the last one added is valid, but
that one cannot be selected because all other logic in the sound panel
assumes the first hit in `get_iter` is valid. This breaks sound input
selection then.
The fix is easy; only add a new list entry if none with that id exists.
Sound devices may not have an origin, in that case we ended up with a
trailing " - " in the label which should not be there. Add a check that
the origin is non-null and not an empty string and only show the devices
description otherwise.