[Feature] Use different thumbs directory for an external library.

The feature I'm in the process of (re)waiting for thumbnails/face detection to run against a massive external library for a second time, just so a second user can name different faces. It has occurred to me that a lot of time (and disk space) could be saved if external libraries used a different thumbs directory, particularly if the thumbnail files are based on disk path/filename. Because all users share the same thumbnail/face recognition settings, If another user shares the same external library as another user , there should be zero need to regenerate the thumbnails/face detection/etc. It's all the same info. Long term, it would be nice if different users had the option of doing different face recognition settings. I have problems in particular with 4 of my relatives getting blended, and so I need to crank down my max recognition distance. Someone else might not care about those 4 particular people and would be happier with a higher recognition dista…