thumbnails: allow choosing between random object in folder or first object in folder

Currently in the PDF Group View if a contained item is a folder, the thumbnail for that folder is randomly chosen from the folder's contained image or file objects.

However this means that what gets shown for thumbnails is unpredictable.

Would be nice to allow choosing between the current behaviour and always taking the thumbnail from the first image or file in the contained folder (maybe recursively?)

Start at the PDF Group View thumbnail-selection entry point and trace how a contained folder's image or file object is chosen. Determine where the random behavior is implemented, then define how a selectable first-object option should behave, including whether recursive folders are supported; add coverage showing both choices.

Written by the indexing model from the issue text.