[Stunner] Arrows are incorrectly positioned when save-open diagram

1. Add a `Decision Node`
2. Add a `Input Data Node` bellow `Decision Node`
3. Connect `Input Data Node` to `Decision Node`
4. Save
5. Close
6. Open againThe arrow from `Input Data Node` to `Decision Node` will be at right of the `Decision Node` and if you move the `Input Data Node`, the arrow is not repositioned.
I noticed this in DMN Showcase and in drools-wb.
And if you add a new `Decision Node` and a new `Input Data Node`, without save and close, it behaves as expected.