x11/lightdm-gtk-greeter: update to 2.0.8

[madpilot]

Summary

This is an update to lightdm-gtk-greeter 2.0.8, the project also moved to github, I reflect this in pkg-descr.

This update also changes dependency from exo-csource in libexo to xdt-csource in xfce4-dev-tools.
SUch change will be needed with future xfce releases since libexo does not include exo-csource anymore.

Test Plan

Building fine in poudriere on supported FreeBSD versions and running fine on my machines.

[woodsb02]

Thanks for this Guido.
LGTM!