nested-grid does not support :src prop like other components

Most re-com components support the optional `:src` property. This is an debugging aid when inspecting the dom and trying to map to actual app code.

e.g.
```clojure
(ns your-ns
 (:require [re-com.core :as rc :refer [at]]))
...
[rc/h-box
 :src (at)
 :children [ .... ]]
```

The nested-grid should also support this as an optional property
