Refactor thumbnails

[OfficiallyM]

Thumbnail generation is slow and there's been one known case of it crashing the game (suspect it took too long and Unity killed the process)

Multithread the generation to improve performance

[OfficiallyM]

Requires more thought.

Queue all needed thumbnails, multithreaded generate in chunks, provide all thumbnails to UI?

Currently, thumbnail is loaded when looping all items so needs to either be available at that point or be added to the items list post generation

[OfficiallyM]

Attempt to find which items don't have thumbnails and create those, no point regenerating all the thumbnails every time. Just add a way of clearing the thumbnail cache manually from the UI.

Could still be worth generating the thumbnails multithreaded in chunks to improve initial load performance

[OfficiallyM]

Changed to only create missing in 3bf9d62
Also added a button in misc to force refresh the cache

[OfficiallyM]

Delaying rest until next version