`Actor` owned by `ConcernUsage` or `ConcernDefinition` is not properly handled

[jerome-obeo]

ConcernUsage is used but same is observed with ConcernDefinition.

1. Create a ConcernUsage
2. Create an Actor (without opening the actors compartment).

The Actor graphical node is created on the diagram, but there is no edge to link it to the ConcernUsage.

1. Create a ConcernUsage
2. Open the actors compartment
3. Create an Actor
4. Show/Hide > Show content as Tree

The actors compartment is well collapsed but the Actor graphical node is not visible on the diagram in Tree mode.