Investigate whether we can show test diff dialogs as editor tabs

[enikao]

Follow-up to #1339

Currently, if we find a difference in a test and show a diff view, the diff view opens as separate window. We might get lots of windows if we run lots of tests. The workflow would be a lot smoother if we could open the diff view as tab, not as new window.

NB: The git diff view went through the same change some time ago -- it used to show up as separate window, but now as editor tab.

[alexanderpann]

For comparing two nodes via the Compare Two nodes action the StructDifferenceDialog is used which we also use in our language. You can of course use the diff dialog in a separate tab:

list<DiffContent> contents = new arraylist<DiffContent>; 
contents.add(new  ModelDiffContent(modelA)); 
contents.add(new  ModelDiffContent(modelB)); 
list<string> titles = new arraylist<string>; 
titles.add("Left"); 
titles.add("Right"); 
DiffRequest request = new  SimpleDiffRequest("My Diff", contents, titles); 
DiffManager.getInstance().showDiff(ideaProject, request);

The problem is that this viewer in contrast to the struct difference dialog assumes that in both models the node IDs are the same, which is an issue for us: