nixos/xdg/icons: Install hicolor-icon-theme

While the package contains no icons, it includes an `index.theme` file
describing directories where toolkits should look for icons installed by apps.

nixos/modules/config/xdg/icons.nix

@@ -1,4 +1,4 @@
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 
 with lib;
 {
@@ -23,6 +23,12 @@ with lib;
       "/share/pixmaps"
     ];
 
+    environment.systemPackages = [
+      # Empty icon theme that contains index.theme file describing directories
+      # where toolkits should look for icons installed by apps.
+      pkgs.hicolor-icon-theme
+    ];
+
     # libXcursor looks for cursors in XCURSOR_PATH
     # it mostly follows the spec for icons
     # See: https://www.x.org/releases/current/doc/man/man3/Xcursor.3.xhtml Themes