dock: allow setting tile-types

You can create spacer tiles in the dock by passing empty tile-data with
specific tile-types

modules/system/defaults/dock.nix

@@ -131,9 +131,16 @@ in {
         Persistent applications in the dock.
       '';
       apply = value:
+        let
+          tileTypes = ["spacer-tile" "small-spacer-tile"];
+        in
         if !(isList value)
         then value
-        else map (app: { tile-data = { file-data = { _CFURLString = app; _CFURLStringType = 0; }; }; }) value;
+        else map (app:
+          if elem app tileTypes
+          then { tile-data = {}; tile-type = app; }
+          else { tile-data = { file-data = { _CFURLString = app; _CFURLStringType = 0; }; }; }
+        ) value;
     };
 
     system.defaults.dock.persistent-others = mkOption {

tests/system-defaults-write.nix

@@ -49,7 +49,7 @@
   system.defaults.dock.appswitcher-all-displays = false;
   system.defaults.dock.autohide-delay = 0.24;
   system.defaults.dock.orientation = "left";
-  system.defaults.dock.persistent-apps = ["MyApp.app" "Cool.app"];
+  system.defaults.dock.persistent-apps = ["MyApp.app" "Cool.app" "small-spacer-tile" "spacer-tile"];
   system.defaults.dock.persistent-others = ["~/Documents" "~/Downloads/file.txt"];
   system.defaults.dock.scroll-to-open = false;
   system.defaults.finder.AppleShowAllFiles = true;