Demonstrate usage of ModelService in the examples #60

This is a follow-up to issue #46.

The examples currently use the ModelServerClient to perform functions such as fetching model content and perhaps performing validation or edits. These examples should be updated to use the corresponding ModelService APIs, instead, because this is now the recommended usage for a few reasons:

• the ModelServerClient is a direct connection to the Java server and so does not invoke any of the business logic extensions provided by plug-ins in the Node server
• the ModelService is intended to evolve certain local performance improvements over time, such as caching model data to avoid round-trips to the Java server