yabar: add module

To make the configuration of `yabar` more pleasant and easier to
validate, a NixOS module will be quite helpful.

An example config could look like this:

```
{
  programs.yabar = {
    enable = true;
    bars.top.indicators.exec = "YA_DATE";
  };
}
```

The module adds a user-controlled systemd service which runs `yabar` after
starting up X.

nixos/tests/yabar.nix

@@ -0,0 +1,25 @@
+import ./make-test.nix ({ pkgs, lib }:
+
+with lib;
+
+{
+  name = "yabar";
+  meta = with pkgs.stdenv.lib.maintainers; {
+    maintainers = [ ma27 ];
+  };
+
+  nodes.yabar = {
+    imports = [ ./common/x11.nix ./common/user-account.nix ];
+
+    services.xserver.displayManager.auto.user = "bob";
+
+    programs.yabar.enable = true;
+  };
+
+  testScript = ''
+    $yabar->start;
+    $yabar->waitForX;
+
+    $yabar->waitForUnit("yabar.service", "bob");
+  '';
+})