This preserves the stereo-ness of files that were imported or recorded in stereo.
ToDo (maybe someday): provide a disclosure triangle that exposes the individual channels in a multichannel region
Duplicate the remove_unsued_region action in the Session->Cleanup menu, for discoverability.
Region list should use the same Region actions as the canvas...
OK, now resolve the change in use-cases of Region List and Source List. tentative.
Rough-in remove_selected_sources (Delete in Sources list).
Column tweaks.
