feat(audio): use cosmic-settings-daemon varlink interface

[mmstick]

Must be merged and tested alongside pop-os/cosmic-settings-daemon#135

• Integrates with Add varlink settings daemon interfaces for handling audio with pipewire cosmic-settings-daemon#135
  • Uses varlink to switch sink and source nodes, set volume and mute status, and to watch for device changes
  • The settings-daemon must be running in the background with the new varlink socket though

The audio applet previously connected directly to pipewire with a crate that it shared with cosmic-settings, and that contains quite a few issues that the settings-daemon implementation has resolved.

---

• I have disclosed use of any AI generated code in my commit messages.
  • If you are using an LLM, and do not fully understand the changes it is making to the code base, do not create a PR.
  • In our experience, AI generated code often results in overly complex code that lacks enough context for a proper fix or feature inclusion. This results in considerably longer code reviews. Due to this, AI authored or partially authored PRs may be closed without comment.
• I understand these changes in full and will be able to respond to review comments.
• My change is accurately described in the commit message.
• My contribution is tested and working as described.
• I have read the Developer Certificate of Origin and certify my contribution under its conditions.

[jacobgkau]

This branch has conflicts.