add -c option to qsh

Much like the shell, this option just runs the specified qtile command and
exits, e.g.:

qsh -c 'help'
qsh -c 'debug'
qsh -c 'eval("self.screens.index(self.currentScreen)")'
Remove outdated comment

As of 5c913fc77728f975ae22c53e21fff3fdae894686, this is done (boxes here means
group boxes).