force thumbnail generation for a single file
deleting the entire cache is overkill ; in thumbnails dir files are not referenced by path but by an obscure hash (looks like md5, but it seems it is not)
sometimes a user might want to force thumbnail generation for a single file that is above the configured size for automatic thumbnail generation, or on a networked device
it would be really useful to have a menu entry (or manual command) to "force (re)generation of the thumbnail for this/these files", along with messages in stderr that would verbosely tell what went wrong and/or which library/command is used to generate the thumbnail (including the use of an in-file thumbnail)