Storing results of previous checks in a history list instead of just clearing them

[VioletGiraffe]

So what I have in mind is a "History" combobox, which would default to "Latest check", but you can go back to any previous analysis result if you so desire. That way we don't need to ask whether you want to run new analysis or not - we'll just run it, and if you want to go back and review previous results (like you were checking a project, then editing some files and saving them) - you can do so. Of course, while you're editing files older problems will become obsolete (clicking them should not crash the add-in), but you'll still be able to review the messages if you want.

[VioletGiraffe]

Implementing this should also deal with #43 - switch to "Latest check" every time a check is completed.