Elements from standard libraries can be edited by dropping them on diagrams

[gdaniel]

It is possible to drop an element from a standard library on a diagram (tested with General View).
It is then possible to trigger a direct edit on the element, allowing to change its name, multiplicity, typing, etc.

Dropping these elements on a diagram should be forbidden, or they shouldn't be editable in diagrams. The second option is probably tricky because we need to prevent direct edit but also node tools, and some edge tools that can update the target object.