Add DM screenshots and lightdm slick-greeter config

- Wire sddm, lightdm, lxdm, greetd, ly screenshots into packagechooser_dm.conf
- Add new antergos-lightdm-config package (lightdm.conf + slick-greeter.conf)
- LightDM netinstall now includes lightdm-slick-greeter and antergos-lightdm-config

packages/antergos-lightdm-config/slick-greeter.conf

@@ -0,0 +1,12 @@
+[Greeter]
+background=/usr/share/backgrounds/antergos/antergos-wallpaper.png
+background-color=#1E3A5F
+draw-user-backgrounds=false
+draw-grid=true
+theme-name=Adwaita
+icon-theme-name=Adwaita
+font-name=Noto Sans 10.5
+show-clock=true
+show-hostname=true
+show-power=true
+enable-hidpi=auto