PR# 11669 A diagram can't be cancelled, locks UI

I picked what I thought was a harmless subcluster and dropped into the "target" button/hole thing in the diagram tool. It immediately started generating a diagram for all 15,000 classes in the entire cluster heirarchy, going up to the root. 

The UI is locked, so there's no way of stopping the diagram tool.

The diagram tool should probably halt automatically, or ask for confirmation if more than say, 20 objects are being rendered, because diagrams with more than 20 objects aren't going to be all that useful anyway.

If I pick a subcluster with 15 objects in it, it would be nice if the diagram tool constrained itself to the subcluster rather than greedily traversing to the root and going off into the land of nod while trying to eat the entire planet.