Support shared thumbnail repositories of the freedesktop.org thumbnail spec
https://specifications.freedesktop.org/thumbnail-spec/thumbnail-spec-latest.html#SHARED
In the Loading thumbnails from a shared thumbnail repository
section, it is written that thumbnails should be generated only if a shared repository with the requested thumbnail doesn't exist.
This MR complements thunar!126 (merged)
Draft notice: a Thumb::time check is missing
Edited by Sergios - Anestis Kefalidis