Manually detect/outline faces

[z-designs]

Describe the enhancement you'd like
An option to manually select an area on an image with a face or some way to set a mark that there is a face.

Describe why this will benefit the LibrePhotos
If this could be implemented and connected with the facial recognicion to help it train, that could improme the accuracy of it.

[derneuere]

Looks similar to #431