This MR does the following to the sliders in the sound panel:
- Fixes the bad practice of subclassing from
GtkBox
by subclassing fromGtkWidget
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)