Remove implicitly generated root class

[larsk21]

Currently, a developer can create a single root class explicitly using create root. If this is not done, a root class is implicitly created, in which all other non-contained classes are contained.

This is confusing, as it adds a class to the view type / view, which might not be needed/wanted, and clutters the view type visualization.

Therefore, we should stop implicitly generating a root class and instead place all non-contained classes and instances directly in the EMF resource contents. While not allowed by the Eclipse modeler, the API seems to support this.