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/calamares-branding-antergos-next/packagechooser_dm.conf

@@ -8,7 +8,7 @@ items:
     - id: SDDM
       name: "SDDM"
       description: "QML-based display manager (KDE default)"
-      screenshot: ""
+      screenshot: "sddm-screenshot.png"
       netinstall:
         name: "SDDM"
         selected: true
@@ -20,7 +20,7 @@ items:
     - id: LightDM
       name: "LightDM"
       description: "Cross-desktop display manager (GTK)"
-      screenshot: ""
+      screenshot: "lightdm-screenshot.png"
       netinstall:
         name: "LightDM"
         selected: true
@@ -28,10 +28,12 @@ items:
         packages:
           - lightdm
           - lightdm-dinit
+          - lightdm-slick-greeter
+          - antergos-lightdm-config
     - id: LXDM
       name: "LXDM"
       description: "Lightweight display manager (LXDE)"
-      screenshot: ""
+      screenshot: "lxdm-screenshot.png"
       netinstall:
         name: "LXDM"
         selected: true
@@ -42,7 +44,7 @@ items:
     - id: greetd
       name: "greetd"
       description: "Minimal display manager (recommended for COSMIC)"
-      screenshot: ""
+      screenshot: "greetd-screenshot.png"
       netinstall:
         name: "greetd"
         selected: true
@@ -53,7 +55,7 @@ items:
     - id: LY
       name: "LY"
       description: "Minimal TUI display manager"
-      screenshot: ""
+      screenshot: "ly-screenshot.png"
       netinstall:
         name: "LY"
         selected: true