nixos/plasma5: add spectacle default

Fixes https://github.com/NixOS/nixpkgs/issues/99680.

In the future I think it would be nice if plasma5 could assume
an approach for an interface we have in the gnome3 module [0].
Notably being able to exclude packages with an option from
the default environment and having a default environment that
is useful to the average user. See [1], currently plasma5 defaults
are very "hard core hacker" with the most bare bones setup.

[0]: https://github.com/NixOS/nixpkgs/issues/67310
[1]: https://github.com/NixOS/nixpkgs/issues/67310#issuecomment-524649585

nixos/modules/services/x11/desktop-managers/plasma5.nix

@@ -274,6 +274,7 @@ in
           plasma-browser-integration
           plasma-integration
           polkit-kde-agent
+          spectacle
           systemsettings
 
           plasma-desktop