Project restructure

[kthoms]

There are some branches that provide additional Job configurations via Jenkinsfiles now, e.g. check-versions and set-xtext-version. For each Job there should be a subfolder, and the general library scripts should be at a proper place. I'd expect a Maven like structue, i.e. nothing in the root, but have folders like
src/main/groovy
src/main/jenkins
src/test/groovy
src/test/resources