Added error message when mono track is selected.

git-svn-id: http://audacity.googlecode.com/svn/audacity-src/trunk@13417 47890b92-0858-11df-a26f-8b716316a5bc
Add new SelectedRegion.h to Linux build



git-svn-id: http://audacity.googlecode.com/svn/audacity-src/trunk@13418 47890b92-0858-11df-a26f-8b716316a5bc
Adding SelectedRegion.h to OSX build



git-svn-id: http://audacity.googlecode.com/svn/audacity-src/trunk@13419 47890b92-0858-11df-a26f-8b716316a5bc