Face detection for the annotation dialog

This adds face detection using libkface from Digikam and automatic area creation to the annotation dialog.

I played around with face detection and face recognition. My current opinion is that we can add face *detection* without a problem but not face *recognition*.

Using the proposed patch here, the face detection feature will be built if libkface is there and simply be left out if not. So it doesn't hurt, does not add additional dependencies, does not interfere with anything else and is a nice feature to have, as one does not have to draw faces rects by hand and the detection process only takes some seconds.

Not so for face *recognition*. I didn't change KPA code to try this out, I just used the "recognize" test program included in the libkface sources. I simply let KPA detect faces on a test set with several images of four persons and saved the found sub-images to separate files, so that I had the very data KPA would use to recognize faces.

The recognition process did work somewhat but we *really* can't use it as in the "recognize" program. Training it with four sample faces of the four persons, it already could identify a lot of the images. The training and the recognition process were still quite fast, they only took a few seconds. Adding the fourth set of training images already took about 15 minutes per face, the recognition process about 90 seconds per image, not speaking of the database being grown to about 300 MB with that few datasets.

Well, somehow the Digikam folks do it in a different way I think ;-) So, as said, this will be much more work as the detection.

Anyways, the detection and automatic area creation would be a nice feature.