fnott: add D-Bus service file

This is needed to make fnott bus-activatable. Otherwise it won't be
started when a bus request is made [0]. Done in the same way as e.g.
dunst sets that up [1,2].

[0] https://www.freedesktop.org/software/systemd/man/systemd.service.html#id-1.10.6
[1] https://github.com/nix-community/home-manager/blob/master/modules/services/dunst.nix#L139
[2] https://github.com/dunst-project/dunst/blob/master/org.knopwob.dunst.service.in

modules/services/fnott.nix

@@ -107,6 +107,13 @@ in {
       };
     };
 
+    xdg.dataFile."dbus-1/services/fnott.service".text = ''
+      [D-BUS Service]
+      Name=org.freedesktop.Notifications
+      Exec=${cfg.package}/bin/fnott
+      SystemdService=fnott.service
+    '';
+
     xdg.configFile."fnott/fnott.ini".source =
       iniFormat.generate "fnott.ini" cfg.settings;
   };

tests/modules/services/fnott/systemd-user-dbus-service-expected.service

@@ -0,0 +1,4 @@
+[D-BUS Service]
+Name=org.freedesktop.Notifications
+Exec=@fnott@/bin/fnott
+SystemdService=fnott.service

tests/modules/services/fnott/systemd-user-service.nix

@@ -11,6 +11,10 @@
       assertFileContent \
         home-files/.config/systemd/user/fnott.service \
         ${./systemd-user-service-expected.service}
+
+      assertFileContent \
+        home-files/.local/share/dbus-1/services/fnott.service \
+        ${./systemd-user-dbus-service-expected.service}
     '';
   };
 }