fix(calamares): sync LightDM group to branding config, add zenity to live

- packagechooser_dm.conf (offline + online copies): LightDM group now
  installs lightdm-slick-greeter + antergos-lightdm-config and shows a
  screenshot. The live-overlay was overriding the branding package's
  config with a bare LightDM group (no greeter), so displaymanager's
  greeter_setup() aborted Xfce+LightDM online installs.
- profile.yaml: add zenity to rootfs packages. antergos-offline-install
  uses zenity exclusively but only yad was installed, so the offline
  installer silently did nothing on double-click.

iso-profiles/antergos/live-overlay/etc/calamares-online/modules/packagechooser_dm.conf

@@ -26,7 +26,7 @@ items:
     - id: LightDM
       name: lightdm
       description: "Cross-desktop display manager (GTK)"
-      screenshot: ""
+      screenshot: "lightdm-screenshot.png"
       netinstall:
         name: "LightDM"
         selected: true
@@ -34,6 +34,8 @@ items:
         packages:
           - lightdm
           - lightdm-dinit
+          - lightdm-slick-greeter
+          - antergos-lightdm-config
 
     - id: LXDM
       name: lxdm

iso-profiles/antergos/live-overlay/etc/calamares/modules/packagechooser_dm.conf

@@ -26,7 +26,7 @@ items:
     - id: LightDM
       name: lightdm
       description: "Cross-desktop display manager (GTK)"
-      screenshot: ""
+      screenshot: "lightdm-screenshot.png"
       netinstall:
         name: "LightDM"
         selected: true
@@ -34,6 +34,8 @@ items:
         packages:
           - lightdm
           - lightdm-dinit
+          - lightdm-slick-greeter
+          - antergos-lightdm-config
 
     - id: LXDM
       name: lxdm

iso-profiles/antergos/profile.yaml

@@ -28,6 +28,7 @@ rootfs:
     - calamares
     - python-toml
     - calamares-branding-antergos-next
+    - zenity
     - antergos-welcome
     - yay
     - antergos-next-desktop-settings