[Copy-Paste][ModelExplorer][Diagram] Issues found with copy/paste from ModelExplorer

This issue deals with the copy/paste of elements from the ModelExplorer

Identified Diagram issues :

• duplicated diagrams doesn't always have Diagram#element
• the graphical diagram context (previously called 'owner') is used as semantic diagram content ('element')

This issue only deals about the managment of Diagram#element and Diagram#owner.

We will fill other issues for the managment of the diagram contents after a copy/paste.