[CompositeStructureDiagram] BehavioredClassifier contained by nestedClassifier/classifierBehavior/ownedBehavior references should be represented on diagram

Actually, only activities contained by nestedClassifier reference can be represented on CompositeStructure diagram and other BehavioredClassifier can be only represented from reference ownedBehavior.

BehavioredClassifier contained by nestedClassifier /classifierBehavior/ownedBehavior references should be represented on diagram (for example, they can be represented on diagram by using semantic Drag&Drop from explorer view)

Moreover, Activity creation should use ownedBehavior reference like other BehavioredClassifiers on diagram.

(see https://bugs.eclipse.org/bugs/show_bug.cgi?id=582932 for the same Papyrus Desktop issue)

Steps to reproduce:

1. Create a new Papyrus-Web Composite Structure Diagram
2. from the Explorer view, create an Activity inside a Class. This new activity must be referenced by the feature ownedBehavior.
3. try to drop this Activity in the Diagram -> it doesn't work.