Refactor import/export for datamodels and subtrees
- needs some rework to be clearer
- versioning?
- import subtree with element name twice in subtree only allows to select one (event example)
(from redmine: issue id 999, created on 2018-02-19tgradl)
(from redmine: issue id 999, created on 2018-02-19tgradl)