player/external_files: fix --cover-art-whitelist priority

Since cb56c2f888dbe86dbc38839c32684d54ea93c63e, the priority of
--cover-art-auto and --cover-art-whitelist is unclear. Depending on the
number of elements in --cover-art-whitelist, the priority of
--cover-art-whitelist files can be lower, equal, or higher than
--cover-art-auto matches, creating unpredictable behavior.

Fix this by making --cover-art-whitelist priority always between "fuzzy"
and "all" priority. test_cover_filename now uses bstr_find_in_list0 to
return a negative priority value depending on the position in list.
The "all" priority is set to INT_MIN. Modify filter_subidx to use 0
priority as removal marker, since all files in the external files list
already have non-zero priority.