Monitor : allow to monitor more than one track

[moonshaadow]

Allow to monitor more than one track. It could be useful in many case.

• It could be done by CTRL + click

• Or simply by clicking on MON to add another track to the monitor output. But Monitor : Can't disable  #78 has to be solved first.

Problems that I see :

• What's happening when monitoring together an output and a track which is routed to this output ? Even if I don't see obvious reason to do that, we should avoid that case, because we don't want to monitor twice some tracks.
  Maybe allow to monitor only inputs or outputs at once, but never inputs and outputs together

• To allow the monitoring of many tracks brings the need of a button to clear all MON.

[SpotlightKid]

You can already use a normal output channel to monitor several input channels, by activating solo ("S") on the control group for that output in all channels to be monitored.

Doesn't work for monitoring output channels, though, but I think that's a rare requirement.