Implement schema editor

- based on GTF ui

• test with preexisting data (e.g. imported); should also allow design from scratch

(from redmine: issue id 296, created on 2018-01-22tgradl, closed on 2018-01-22)

• Relations:
  • relates #298
  • relates #300
  • relates #302
  • relates #303
  • relates #308
  • duplicates #328 (closed)