Adding projects to a workspace took one folder at a time. Choosing a folder that is not itself a project now scans below it, up to four levels and a shared entry budget, without following symlinks or descending into a project and skipping dependency, output and hidden directories. The projects found appear in a checklist with linked ones disabled; a folder with none is added as before. The scan is cancellable and runs in both the create and manage dialogs.
Fixes#2102