sound: Simplify sliders and remove marks

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

This MR does the following to the sliders in the sound panel:

  • Fixes the bad practice of subclassing from GtkBox by subclassing from GtkWidget instead
  • Removes the marks from the balance and face sliders (to match the mockup in #2046)
  • Cleanups the XMLs by removing properties that are already set to their default values
  • Enables the "origin" to the subwoofer slider (it's the filled blue part)





Edited by Marco Melorio

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