Create a new class, which performs all thumbnailing code.
This is mostly code reshuffling. Thumbnails are extracted
either from a cache or thumbnail calculation is started in
a worker thread.
Since getHashedImage() is called from a worker thread it
makes no sense to call subfunctions in yet another worker
thread. Remove these calls.
In contrast to the previous code, on error the background
thread produces a failure image, but it is not yet shown.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
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