ubuntu 16.04 add root to login window
http://askubuntu.com/questions/451950/how-to-configure-lightdm-to-allow-manual-logins-in-ubuntu-14-04


/etc/lightdm/lightdm.conf
or 
/etc/init/lightdm.conf
or 
/usr/share/lightdm/lightdm.conf.d

add line
greeter-show-manual-login=true
