Add gvfs dbus service and gvfs gio modules to gnome 3

Closes #2746

nixos/modules/module-list.nix

@@ -107,6 +107,7 @@
   ./services/desktops/gnome3/gnome-online-accounts.nix
   ./services/desktops/gnome3/gnome-online-miners.nix
   ./services/desktops/gnome3/gnome-user-share.nix
+  ./services/desktops/gnome3/gvfs.nix
   ./services/desktops/gnome3/seahorse.nix
   ./services/desktops/gnome3/sushi.nix
   ./services/desktops/gnome3/tracker.nix

nixos/modules/services/desktops/gnome3/gvfs.nix

@@ -0,0 +1,42 @@
+# gvfs backends
+
+{ config, pkgs, ... }:
+
+with pkgs.lib;
+
+let
+  gnome3 = config.environment.gnome3.packageSet;
+in
+{
+
+  ###### interface
+
+  options = {
+
+    services.gnome3.gvfs = {
+
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether to enable gvfs backends, userspace virtual filesystem used
+          by GNOME components via D-Bus.
+        '';
+      };
+
+    };
+
+  };
+
+
+  ###### implementation
+
+  config = mkIf config.services.gnome3.gvfs.enable {
+
+    environment.systemPackages = [ gnome3.gvfs ];
+
+    services.dbus.packages = [ gnome3.gvfs ];
+
+  };
+
+}

nixos/modules/services/x11/desktop-managers/gnome3.nix

@@ -63,6 +63,7 @@ in {
     services.gnome3.gnome-keyring.enable = true;
     services.gnome3.gnome-online-accounts.enable = mkDefault true;
     services.gnome3.gnome-user-share.enable = mkDefault true;
+    services.gnome3.gvfs.enable = true;
     services.gnome3.seahorse.enable = mkDefault true;
     services.gnome3.sushi.enable = mkDefault true;
     services.gnome3.tracker.enable = mkDefault true;
@@ -101,7 +102,8 @@ in {
       };
 
     environment.variables.GIO_EXTRA_MODULES = [ "${gnome3.dconf}/lib/gio/modules"
-                                                "${gnome3.glib_networking}/lib/gio/modules" ];
+                                                "${gnome3.glib_networking}/lib/gio/modules"
+                                                "${gnome3.gvfs}/lib/gio/modules" ];
     environment.systemPackages =
       [ pkgs.desktop_file_utils
         gnome3.glib_networking