Change image hash compare algorithm and add multithreading

[qarmin]

Closes #679
Closes #761
Helps #512

Algorithm performance for all similarity options should be very similar to fast compare option which become unnecessary and was removed.

Results should be a little worse than with previous algorithm, but scan speed boost especially with bigger similarities, should be clearly visible

Due using multi-threading, checking should be a lot of faster with bigger amount of files(0 similarity is special situation in which comparing images should happen almost instant), since most of operations are calculated on memory/CPU(without any disk operations)

Due removing Similarity enum, image cache file is not longer backward compatible, but still json config can be exported, changed manually and later imported to app(read docs for more info)

The only problem that I see is that I can't really test performance, because with ~8K images which I'm able to scan

With my testing on 20K images, new algorithm(4/8 processor) is a lot of faster than normal scan, but slightly less performant than fast compare

--- Similarity 20

New Normal 9
Old Fast Compare 6
Old Normal 106

--- Similarity 2

New Normal 2
Old Fast Compare 2
Old Normal 2

[chchia]

Due removing Similarity enum, image cache file is not longer backward compatible

that means i will have to rehash a total 1.8millions images again, that will be too bad, could you remain the option to use the old method? it will be too much for me to rehash 1.8 millions files as i store my file in online storage....

[qarmin]

From the app perspective, using integer instead enum is only cosmetic change(enum with 1 field doesn't have much sense), but maybe this is not worth breaking changes

[chchia]

i was thinking maybe you can consider within folder comparison.

example the user may have hundreds of folders, but the picture in each folder was actually categorize, example i might shooting the same scene for 30 photos with very minor different, and all those folder is put under a single folder.

so if there is a option let say "Compare images within the same folder only", with this i believe we can get a significant merit of:

1. shorter comparison time as we dont have to compare 1 file to all folder, but instead only image within the same folder.

2. with this, i can set the Similarity to even like 20, those same scene picture are more likely to be categorize as same. otherwise if comparing all photo it is more likely to see mistake.