Use CNN face detector as fallback

[m8nmueller]

Since dlib's frontal_face_detector didn't work well for my photos (admittedly not only clear frontal views), I tried to use the CNN detector, that is also present in dlib. It is much slower, though, so I used the given compromise: try the traditional face detector first and fall back to the CNN detector.

For the small subset of images of mine, this created a huge stability advantage (didn't work before, worked afterwards) for as low a time cost as possible. I didn't experience problems of incompatibilities between the detected points.