Audio Preview: Add a centerline and change color palette of the visualisation
- This MR visually separates the left / right audio channels by inserting a gap. Alternative a center line between the peaks can be drawn instead.
- Additionally it changes the color pallet of the visualization, which now completely relies on the GtkScale progress color (which turned out to be equivalent to
theme_selected_bg_color
for the tested themes) instead of the link color and an arbitrary chosen gray.