Audio: keep the channels list in sink with pulse signals
Audio: Update the pupup when channels got added/removed.

This still has some issues....
Audio: place new channels on top of the list
Audio: do not show the channel icon

...I do not like it :P
Audio: update the sliders on volume/mute changes
Audio: avoid setting the slider value if we are dragging it
Audio: manage the volume as a single int
Audio: keep the default (fallback) sink on top of the list

This way the main speaker works on the fallback sink
Audio: also update all the speakers on volume/mute changes
Audio: middle-click on the sliders to toggle mute