Impr sugg: instrument editor: instruments solo setting

[GoogleCodeExporter]

"solo" checkbox: mute all other instruments in the output mix, except for the 
other instruments that are already in solo mode.

"clear solo" button: clear all "solo" checkboxes of all instruments and restore 
normal output.

Original issue reported on code.google.com by [email protected] on 17 Aug 2014 at 2:47