nixos-artwork: add file path attributes

This makes things so much easier, and we install to
the path that both gnome-backgrounds and
elementary-wallpapers install to.

pkgs/data/misc/nixos-artwork/wallpapers.nix

@@ -3,12 +3,18 @@
 let
   mkNixBackground = { name, src, description }:
 
-    stdenv.mkDerivation {
+  let
+    pkg = stdenv.mkDerivation {
       inherit name src;
 
       dontUnpack = true;
 
       installPhase = ''
+        # GNOME
+        mkdir -p $out/share/backgrounds/nixos
+        ln -s $src $out/share/backgrounds/nixos/${src.name}
+
+        # TODO: is this path still needed?
         mkdir -p $out/share/artwork/gnome
         ln -s $src $out/share/artwork/gnome/${src.name}
 
@@ -22,6 +28,11 @@ X-KDE-PluginInfo-Name=${name}
 _EOF
       '';
 
+      passthru = {
+        gnomeFilePath = "${pkg}/share/backgrounds/nixos/${src.name}";
+        kdeFilePath = "${pkg}/share/wallpapers/${name}/contents/images/${src.name}";
+      };
+
       meta = with stdenv.lib; {
         inherit description;
         homepage = "https://github.com/NixOS/nixos-artwork";
@@ -29,6 +40,7 @@ _EOF
         platforms = platforms.all;
       };
     };
+in pkg;
 
 in