Multicore support for "Metadata > Update catalog from files"

[raindal]

I use some external programs to manage the metadata of my photo library, e.g. IPTC Keyword tags. After doing this I then update in XnView MP by using the "Metadata > Update catalog from files" command. This takes quite a long time to run, and as far as I can tell seems to be bottlenecked on CPU since it maxes out at ~90% utilization of a single CPU core. I don't think it's bottlenecked on disk or memory bandwidth.

It'd be nice if this could be sped up by taking advantage of additional CPU cores, e.g. by having a bunch of threads all reading the new metadata from disk and then passing that info off to another thread that updates the SQLite database.

(Separately, it would also be nice if the progress bar during this process could be made a bit more accurate. Currently it seems to only track processing of the top level selection of items, e.g. top level directories, rather than incrementally tracking progress for all files that are recursively in scope.)