Prototyped CDT C++ class DragAndDrop support for the sequence diagram and class diagram.

There is only one tree class-element could be DND on the diagram.

Next steps:
- Multiple selection handling
- DND file or folder
- c++ Namespaces support (especially multiple and anonymous)
- Connections handling
- Check auto completion Ctrl+SPACE for the sequence diagram (most likely it works)
- Understanding the UML Model + resources
- How to get information about model element (Get inherited classes, associations, generalizations etc)
- How to setup name-based filters to avoid multiple methods/fields showing instead of really necessarily