Use EXIF metadata to supplement labelling people

[fcimeson]

Describe the enhancement you'd like
Read the PersonInImage tag from exif to supplement labelling people.

Describe why this will benefit the LibrePhotos
ML doesn't always detect the faces in the photo and without a manual override we are at the mercy of the ML (some photos will go unclassified). E.g., the person isn't showing their face, the classifier didn't recognise the face as a face, etc.

Additional context
People labels should be tied to the photo instead of a database, in this way someone could blow away their database and start over again without losing this data.

[Michael2MacDonald]

Look at issue #130
Maybe we should move to that issue and close this one?