This patch aims to improve the usability of fuzzy/duplicates/sketch search.

A sorting option (by similarity) is introduced which can be helpful in fuzzy/sketch search where similar images are searched for a given one. Sorting the resulting images by similarity to the given image is an obviously helpful feature.
It is now possible to drop local files that are images and not present in the Digikam database into the fuzzy search image label and trigger a fuzzy search for this image.

Details:

Sort by similarity:
For sorting by similarity, a new option Sort by similarity with default order descending was introduced. This option is only active if the fuzzy search sidebar is active, i.e. fuzzy/duplicates/sketch search.
To compare two pictures, it is necessary to get their similarity to the original picture. Since this information was only printed to console but not saved in the image info, SAlbum query or the properties,
I do that myself. For every similar picture, a property similarityTo_X with X being the id of the original image and the similarity are stored as image property. When the image with id X is deleted, the property
is removed to keep the DB small. Also, the property is removed from images if the fuzzy/duplicates search is done for image X such that old similarity values are removed. Though the similarity of image X to image Y
is symmetric, I do not want to store both. This would bloat the DB. Only the detected similarities are explicitly stored.

Drag and drop images not yet present in DB for fuzzy searches:
I allow dragging objects over the image label iff the mime type of the event data is a URL list. In this case, it is no DIMG. If the first URL (there is only one in the list) is a local file and can be loaded as QImage, dropping is allowed. The function
loadImage in HaarIface is used since it was already present and has optimisations for specific image formats. In order to not import the image but to be able to find similar pictures, and sort by the similarity, a temporary image id is needed.
I use -1 for this case since this image id cannot be present in DB. Also, everytime a new external image is dropped for fuzzy search, the similarities for the previous dropped image are removed from database to keep the entries up to date.
Moreover, I save the image url of the dropped image in the fuzzy search view. This way, refreshing the similar images due to modification of thresholds does work, too.

Made small tests with valgrind - did not find memory leaks, yet.
Made functionality tests for fuzzy drop. Trying to drag local files that are no images (e.g. pdf, odt, plain text) is not successful as wished.
Made functionality tests for similarity search. Did not find defects.
More tests will be applied in the next days.

BUGS: 302923
BUGS: 320666
FIXED-IN: 5.4.0