Paste as copy doesn't work for grouped items (#2743)

To reproduce:

1. Start a use case diagram

2. Add two use cases

3. Group them

4. Select the group and press Ctrl+C

5. Press ctrl+v

6. In the dialogue box, select Paste as copy

7. Ungroup the copy

8. Select a use case and change its text

The text of the original element will also be changed