Add basic directed acyclic graph data structure

Also make use of this instead of Nixpkgs's strings-with-deps library
in activation script generation.

modules/programs/gnome-terminal.nix

@@ -1,6 +1,7 @@
 { config, lib, pkgs, ... }:
 
 with lib;
+with import ../lib/dag.nix;
 
 let
 
@@ -177,18 +178,19 @@ in
   config = mkIf cfg.enable {
     home.packages = [ pkgs.gnome3.gnome_terminal ];
 
-    home.activation.gnomeTerminal =
+    # The dconf service needs to be installed and prepared.
+    home.activation.gnomeTerminal = dagEntryAfter ["installPackages"] (
       let
         sf = pkgs.writeText "gnome-terminal.ini" (toINI (buildIniSet cfg));
         dconfPath = "/org/gnome/terminal/legacy/";
       in
-        # The dconf service needs to be installed and prepared.
-        stringAfter [ "installPackages" ] ''
+        ''
           if [[ -v DRY_RUN ]]; then
             echo ${pkgs.gnome3.dconf}/bin/dconf load ${dconfPath} "<" ${sf}
           else
             ${pkgs.gnome3.dconf}/bin/dconf load ${dconfPath} < ${sf}
           fi
-        '';
+        ''
+    );
   };
 }