update migration assistent such that one can choose from based on which version a project should be reimigrated.

add migration incident to delete an element inside an array, improve the code of the Graph/Tree-class

the language should now change correctly, especially if the default language of the operating system is german.

This branch makes attributes being Cloneable.
For attributes, the default shallow clone implementation is used.
This is good as long as attributes do not have mutable sub-objects.
Note that VShape objects are not and must not be mutable.

ScenarioElements implement a clone method. Implementations use a
constructor and pass cloned attributes. ScenarioElements are not
Cloneable because they do not user super.clone() which makes a shallow
copy.

- class instead of interface for technical reasons
- use standard implementation of clone
- inherit standard clone method from base class