When gvc gives us an active-input/output-device ID we don't know or sets the ID
to 0, that means there's no device currently active. While this seems unlikely
to be the case without the device getting removed at the same time, it can happen:
When the bluetooth profile gets changed from handset (input+output) to
headphone (output only), the input device remains available, but there's no
more active-input-device anymore.
In this case, we shouldn't leave the active entry on the device combo box as-is,
instead we should set it to NULL to indicate that no input device is set.
This fixes a bug where the input switcher is not updated when there's no
internal microphone but there's a bluetooth headset connected, and the
bluetooth profile gets switched from Handset to Headphone.
We should only listen to "active-input-update" and "active-output-update" in
one place, and then update the combo box from there. This way we can block our
own signal handler for input/output device changes on the combo box, which
we'll do in the next commit.
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.
