Stereo vu meter on Mixer view

[Patrick]

@djenzar done. A nice complement to my bug report:

UI Inconsistency at showing levels in the mixer

Bug Description As this question was coming up: https://backstage.polyend.com/t/how-to-mute-synths-on-t-or-mini/ A short description of the ui inconsistency: samples, midi and synth engine are sound sources. These can go to track 1-8. Synths and Midi can go to track 1-16. In the mixer levels are displayed for track 1-8, you can change volume and mute the track. Synths have their dedicated metering. Problem: Levels on the track meters are displayed, as long as there are samples on this par…