attach/detach by mixer class implementation of alsa-lib mixer API

[Takashi Sakamoto <o-takashi@sakamocchi.jp>]

Hi Jaroslav,

Recent years I'm bothered about unexpected abort of pulseaudio and
pipewire processes when testing user-defined control element set.
They aborts at element removal event.

```
pulseaudio: mixer.c:149: hctl_elem_event_handler: Assertion `bag_empty(bag)' failed.
wireplumber: mixer.c:149: hctl_elem_event_handler: Assertion `bag_empty(bag)' failed.
```

Would I ask your opinion about the design of alsa-lib mixer API?

As long as I investigate, these programs seem to have careless coding as
alsa-lib mixer API application. Both of them attaches an instance of
snd_mixer_elem_t to an instance of snd_hctl_elem_t by calling
snd_mixer_elem_attach() when detecting element addition, but never detach
it even if detecting element removal.

In the case, the link list (=bag) of mixer API internal never becomes empty.
It has link entries as much as the number of registered mixer classes which
attaches snd_mixer_elem_t. Then it hits the assertion.

I think the design of alsa-lib mixer API demands mixer class implementation
to detach at element removal which attached at element addition. But I have
less conviction about it since enough unfamiliar.

I'm glad if receiving your opinion about it.

I wrote test program for the issued behaviour:
 - https://gist.github.com/takaswie/8378fe3bc04652d83428694cd7573bc0

For test, please use sample script in alsa-gobject project:
 - https://github.com/alsa-project/alsa-gobject/blob/master/samples/ctl

The patches for pulseaudio/pipewire are prepared:
 - https://gitlab.freedesktop.org/takaswie/pulseaudio/-/commit/topic/fix-wrong-handling-alsa-ctl-event
 - https://gitlab.freedesktop.org/takaswie/pipewire/-/commits/topic/fix-wrong-handling-alsa-ctl-event


Regards

Takashi Sakamoto