Relationships not sync'ed across diagrams (#360)

1. Create a diagram with Elm descending from Tree.

2. Create another diagram and drag both Elm and Tree to it.

3. In either diagram, delete the generalization relationship and instead have Elm be associated to Tree.

4. View the other diagram. It still shows the old relationship. It's not clear how to refresh it. I tried closing and reopening it, but it still showed the old relationship.