This is accomplished via a new event, "import_task_apply", which is called
right after metadata is applied to newly-imported items.
This change makes chroma REQUIRE a new version (0.6) of pyacoustid. Users with
older versions installed will see complaints about a missing method
"fingerprint_file".
The old "caching"-based approach to fingerprinting was kinda hacky to begin
with. Now, the chroma plugin has an explicit opportunity (in the form of a new
event) to perform its initial fingerprinting and lookup for all tracks. Then,
this information is used explicitly during the autotagging phase rather than
being used transparently through memoization of the lookup function.
