add a volume slider independent of the process

[LFSaw]

An idea that came up during the recent PM workshop (and @sletz pointed out to add an issue here):

add a volume slider to the webIDE independent of the process.

something along the line of

si.bus(outputs(component)) : par (i, outputs(component), _*vol) with h{vol = hslider("vol", 0.1, 0, 1, 0.0001);};