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This cleans the way we signal task completion, reduces contention, and
will allow us to handle multithreading better

So far, we only handle the case where a Media has multiple files

This is already done when adding the track to an album

Instead, have a FOREIGN KEY in those extension tables
This also mandates a media to be provided when creating a
movie/showepisode, and constify the extension getters

This is far from complete, but it's a start

Or fallback to the album artist/unknown artist

While we fixed an issue when the first disc is not the first disc being
analyzed, we broke the case when it actually is.
This should now always work as soon as the tracks have either disc total
or disc number meta set, regardless of the order of the file

We were relying on the idea that the first disc would always be the
first parsed file, which is not true

This allows us not to create a new SQLite connection from a potentially
different VLC thread for each media

Instead of having a per-field SQL request.

Instead, have the caller specify which type they want.
This allows us avoid using those ugly std::static_pointer_cast