nixos/display-managers: Add DesktopNames parameter to generated desktop session files

Some display managers (e.g. SDDM) set the XDG_CURRENT_DESKTOP variable accroding to this parameter.
If this variable is not defined, there will be some problems (e.g. MATE doesn't have icons on the desktop).

Fixes https://github.com/NixOS/nixpkgs/issues/71427

nixos/modules/services/x11/display-managers/default.nix

@@ -427,6 +427,7 @@ in
                     TryExec=${script}
                     Exec=${script}
                     Name=${sessionName}
+                    DesktopNames=${sessionName}
                   '';
                 } // {
                   providedSessions = [ sessionName ];