More intelligent project kit configuration when an old build folder is encountered

Steps to reproduce

1. Use Qt Creator to create a new Qt project. (After this, the Debug build folder should get auto-generated)
2. Close Qt Creator
3. Delete the project's *.user file
4. Open Qt Creator and reload the project

Outcomes
Due to the lack of a *.user file, Qt Creator asks us to configure the project kits. However, the presence of the old build folder causes a duplicate entry:

Suggestion
Don't show a duplicated entry. Rather, ask if the user wants to use the existing folder, delete/replace it, or use a different folder