* Add etc/xdg/lubuntu/lightdm/lightdm-gtk-greeter.conf
* debian/{postinst,prerm}: install/remove alternatives to use our config
as /etc/lightdm/lightdm-gtk-greeter.conf.
* debian/control:
- add depends on lightdm-gtk-greeter >= 1.0.0-0ubuntu2.
- provides lightdm-gtk-greeter-config.