minor improvements to the Face Detection UI (progressmanager)

This merge request implements minor improvements to the face detection UI.

1. A more detailed label is set in the 'FacesDetector.cpp' file.
2. Moreover, it is possible to display the progress of several progress items ('ProgressItem.cpp') now. This can be triggered by right-clicking images and pressing 'Scan for Faces' and re-running this proccess on other images while the old process was not finished. While the processes run correctly and images are scanned for faces, the progress of later proccesses was not shown in the past.

Thanks for considering and your work on Digikam!

(This is a copy of merge request !190 (closed) but targets the master branch instead of the qt5 one)