[digikam] [Bug 320666] Add search of similar images outside digiKam collections [patch]

[Simon Frei]

I use internal mysql and the images are not (exclusively) identical ones
(range 30-100% gives all kinds of "similar" pictures as is to be expected).

I added your snippet and it produces 0/1 outputs like this:
    digikam.database: Similarity of image  77839 in imageLister:  0
    digikam.database: Similarity of image  77845 in imageLister:  1