Stunner - SessionDiagramPreview instances are not discarded

SessionPresenterFactoryImpl creates instances of SessionDiagramPreview. Implementations of SessionDiagramPreview are @Dependent and their life-cycle managed by SessionPresenterFactoryImpl using a ManagedInstance. Great.

However instances continue to @Observe events and since the instances will not be destroyed until SessionPresenterFactoryImpl is destroyed (by virtue of the ManagedInstance contract we end up with lots of unnecessary events.

You can see this behaviour by running a (any will do) Stunner webapp and putting a break-point in SessionPreviewImpl.commandExecutedFired(). Following a fresh launch of the application and creating a new diagram the method will be hit X times. Close the diagram and create another. The method will be hit X*2. Close. Create. X*3 etc.

Acceptance test

1. Scenario from PR comment is fixed.