Adding a custom widget to the network dashboard

As discussed in the recipe titled Creating network-level admin pages back in Chapter 3, User Settings and Administration Pages, WordPress offers a very powerful mode called Network mode, which allows for multiple websites to be served from a single installation of the platform. When creating a plugin, developers need to think if it would make sense for their plugin to offer a dashboard widget that would only be seen in the network administrator's dashboard instead of being seen in individual website dashboards, or if their plugin's scope is really more relevant at each website's level. The following recipe shows how to modify the dashboard widget defined in the previous recipe so that it appears in the network administration panel on network installations, while still appearing in the administration dashboard in single...