Add a vertical mark/line at shell's sound slider which indicates where 100% of volume is, when the option "allow louder than 100%" is enabled at settings

[Thanos Apostolou]

This is about the new changes about sound in Artful, where the volume slider is able to get higher than 100% (when the option is enabled at settings). Add a vertical mark/line at shell's sound slider which indicates where 100% of volume is, when the option "allow louder than 100%" is enabled at settings. This will allow users to know, easily, if their sound quality is reduced (over 100%) or not. Originally, this was mentioned as a comment on this blog: https://didrocks.fr/2017/08/16/ubuntu-gnome-shell-in-artful-day-3/#comment-3474135023

[Didier Roche-Tolomelli (didrocks)]

Adding gnome-settings-daemon as well for the OSD part, if communication needed.