Cleanup Visitor implementation

See merge request !33

Style

Recovery of the old folder structure

For overall consistency

Start working on cpp conversion

Resolve "Update Dependencies"

See merge request !31

Visitor rework

See merge request !30

CopyHandler and the Copyable-Interface are obsolete due to Copy.copy
offering the object-copying functionality with serialization.

Hotfix 3.17.1

See merge request !29