Add logic to handle 3 possible thumbnail names, in following order:
- most exact name derived from content file (same name, with .png extension)
- usual name derived from playlist (usually coming from database)
- shortened name up to first bracket, chopping off region/publisher etc. info
For local file system, names are checked always.
For thumbnail downloads, names are checked each time the item comes up
in the playlist, meaning that it may take going back and forth 3 times
for a thumbnail to appear. However, as a positive change, failed thumbnail
downloads are not repeated for the same playlist, which was not the case
earlier.
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