[ActivityDiagram] Activities contained by classifierBehavior/ownedBehavior references should be represented on diagram

Actually, only activities contained by nestedClassifier reference can be represented on Activity diagram.

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

(see https://bugs.eclipse.org/bugs/show_bug.cgi?id=582902 for the same Papyrus Desktop issue)

Steps to reproduce:

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