sound: Tweak and simplify level bars

Merged Marco Melorio requested to merge melix99/gnome-control-center:sound-level-bars into main

This PR makes the CCLevelBar use a GtkLevelBar internally, which simplifies the internal code quite a bit. Also, it moves the position of the CCLevelBar and the test button of the output and input sections to their respective headers, just like the new mockup in #2046.

These are two cherry-picked commits from !1480 (closed), which are already ready to be merged.





Edited by Marco Melorio

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