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

[Simon Frei]

I played around with the patch. However I did not test/use this feature
previously, so I don't know whether the following was introduced with
this patch or already there before:

In "Duplicates" the similarity in table view seem reasonable mostly. But
when there are identical pictures in a group of similar images, there
sometimes is one picture with 100, the other with 101, sometimes just
one picture with 101 and sometimes all values are <100. I don't see any
pattern when this happens. What does the similarity mean anyway in a
group? Average similarity is intuitively understandable, individual
similarity is unclear to me.

In "Images" I get a binary: It is either 100 or 1.