Renamed elements not reflected as validation errors in wfconfig

[eneufeld]

Suggestion from @martin-fleck-at :
The "references" in the wfconfig file are not real cross references but rather validated strings. As such the language server does not trigger a re-validation of the wfconfig file because it doesn't know that the string it has comes from the changed .coffee file.

I believe we could trigger a build/validation ourselves. However, since that information is not stored by Xtext we would need to track that reference information ourselves. Alternatively we would need to build/validate all wfconfig files if there is a change in a .coffee file just in case one of the files references content from that file. I believe the validation of those files is fast and the files are typically not that large. Still, it wouldn't be the most elegant solution.

[...] we have the following options: real cross references, string references but tracked reference information, build/validation of all wfconfig files.

This is a followup to #275