Support for duplication of model elements in a diagram

[roper79]

The goal is to be able to represent one model element using multiple diagram elements. This is the use case for complex diagrams with a lot of associations where duplication of one model element in several places of the diagram reduces the number of cross-diagram lines.

Implementation:

1. Duplication creation: Context menu item "Duplicate in diagram"
2. Extending the remove feature: "Remove from diagram": this item is enabled just in case there are multiple representations of the model element in the diagram

Further extensions:

• Icon indicating that the element has multiple diagram representations
• Context menu item "Highlight all in diagram" to enable easy locating