lightdm: Add lightdm and lightdm-gtk-greeter

Adds the [LightDM](http://launchpad.net/lightdm) display and login manager, along with the [LightDM GTK Greeter](http://launchpad.net/lightdm-gtk-greeter). A few patches are needed to get things running without any manual intervention - which is just the normal fuss about hard-coded directories. I haven't yet submitted these patches upstream, but plan to.