Add x11/lightdm and x11/lightdm-gtk-greeter.

Lightdm is a display manager that:

• Is cross-desktop - supports different desktops
• Supports different display technologies
• Is lightweight - low memory usage and fast performance
• Has a comprehensive test suite

WWW: https://www.freedesktop.org/wiki/Software/LightDM/

lightdm-gtk-greeter is the reference GTK+ greeter for LightDM.

WWW: https://launchpad.net/lightdm-gtk-greeter

Approved by: adamw (mentor, implicit)