Optimize Theia test application to show all features of V2

[ndoschek]

• Align listener usage to proper v2 usage. In api-test-menu we currently use the ModelServerSubscriptionClient which makes the actual subscription calls confusing, as we pass an undefined listener, but also subscription options (see also #179 Consistently use URI object in APIs #121 (comment)_).
• We should show the request posted by the menu entry (e.g. data like the request url and its request body - command or patch)
• Doublcheck if all relevant v2 use cases are covered
• Doublecheck if we can overall optimize the application for a better overview

Originally posted by @ndoschek in #121 (comment)