Select Custom Thumbnails for Files and Use Sidecar Files for Thumbnails During Hydrus Import

[AKL55]

Discussed this on Discord. It’s similar to issue #1393, which is closed, but not the same.

I would like the option to add a custom thumbnail for a selected file in Hydrus.

I would also like to use a sidecar file containing the thumbnail when importing files into Hydrus. This should be simpler than extracting embedded cover art from video files to use as a thumbnail.

Issue #595 is different.