Better level monitoring

From Buzztard

We need better level meter placement. Right now we do this for all machines:

all: input-level | input-gain | machine | output-level | output-gain

Imho gain is only used for sink-machine right now and there we'd actually like to show the output-level (post gain level) This means we need different ordering depending on machine type:

source   : (input-level) | machine | input-gain | output-level |  output-gain
processor:  input-level  | input-gain | machine | output-level |  output-gain
sink     :  input-level  | input-gain | output-level | machine | (output-gain)

Then the level-meter in the toolbar could listen to output-level (which right now is not available for sinks).