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

[Simon Frei]

@Wolfgang:
You need to download the attached patch to the core directory. There you run
    git apply *patchname*
where *patchname* is the actual name of the patch. Then you continue
building as usual.

I am currently disentangling the changes regarding
https://bugs.kde.org/show_bug.cgi?id=374191 and this bug, because there
is something I would like to suggest on the patch for the former. I will
post the separated pasts as soon as I have them - I hope that is ok.