Use Simple-ML to create stubs

[lars-reimann]

Is your feature request related to a problem? Please describe

Instead of creating stub files directly we could build an AST and let Xtext take care of creating (formatted) source code for us.

Desired solution

Publish de.unibonn.simpleml on Maven Central or GitHub and pull it in as a dependency.

Tasks

• Update tests for stub builders to work on the EMF-Model rather than strings
• Pull de.unibonn.simpleml from Maven instead of having the Jar in the repo (requires that the Simple-ML is public)