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