The following patch introduces a few functions for aggregate project
maintenance:

- project-find-projects-under
  Select a directory with projects to index all at once.
- project-remove-zombie-projects
  Check if all known projects still exist and remove those
  that don't anymore
- project-remove-projects-under
  Remove all projects in a directory (inverse of
  project-find-projects-under).

Especially the last two are useful to maintain a clean project list
without having to manually remove every project one by one.