Problems during re-visualization

[djnemeth]

If one tries to re-visualize a model for which there is an already existing Papyrus diagram in the project's gen folder, the visualization fails. If the corresponding diagram is opened during re-visualization, a ConcurrentModificationException occurs and the gen folder has to be cleaned. Otherwise the process seemingly succeeds, but in reality, only the .di and .notation files disappear from the gen folder. In this case, however, a second re-visualization attempt actually creates and opens the correct diagram. This is only for Papyrus – JS works as intended, but it could be improved too.

It would be great if

1. Papyrus diagrams could be re-visualized without the need of cleaning gen, whether the previously generated diagram is opened or not – here we could simply overwrite the existing diagram;
2. the user could decide whether they wants to overwrite the existing diagram or cancel the visualization – this is for both Papyrus and JS.

It would be even greater if we could append "tabs" to existing diagrams. For example, if one visualizes a class diagram from a model in Papyrus and then decides to add a state machine diagram from the same model, we wouldn't need to replace the whole .di file, but simply append the new diagram to the existing one. The same for JS. As this doesn't look like a trivial task, I'm willing to create a separate issue for it.