Changes

Summary

1. tuned signature;
2. tuned;
3. merged;
4. Added tag Isabelle2018-RC4 for changeset 34b8ff7cb109
5. updated for release;
6. updated screenshot;
7. updated documentation;
8. suppress verit-2016post -- not ready for release;
9. isabelle build options -c -x -B refer to imports_graph;
10. more uniform checks;
11. tuned signature;
12. tuned signature;
13. tuned signature;
14. clarified ignored span / core range: include formal comments, e.g. relevant for error messages from antiquotations;
15. tuned signature;