Deleting a project group does not delete related build output directories

To reproduce:
1. add a project group
2. add a project
3. force a build
4. observe numbered directories in both working-directory and build-output-directory
5. delete the project group

Expected results:
The project group, working copies and build output are all deleted

Actual results:
The project group and working copies are deleted, but the numbered build output directories are left in place

Continuum should delete the build output directories when a project is deleted.

(I did not test deleting a single project, just a project group containing three projects.)