System › Sources & scans gains the Thumbnail location row (D196): Inside
each source or One folder, the folder path, the status line and Review
move…, whose review (D197) lists what is copied, removed and left before
Move thumbnails starts; a refused folder is explained under the field, a
folder of your own asks for a confirmation first. Activity shows a move
and a prune with their own tiles and step line (D198). An unavailable
folder shows a notice on Overview and Sources & scans with Recreate folder
or Use this folder (D199). Thumbnail maintenance offers Remove thumbnails
of removed sources in folder mode (D200), and Move library's review says
where the thumbnails stay (D201).
Closes#320