AlbumTracks now store a genre id, instead of a plaintext genre.

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

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 now only part of the test implementation, and allows us to
remove quite a bit of overhead

Instead of re-computing it dynamically while it's never going to change

As a result, the Media.Duplicate test is obsolete, since a filename
isn't unique in itself. Only the couple (mrl, folder_id) is UNIQUE.
However, UNIQUE doesn't apply when the folder_id foreign key is NULL
Uniqueness has been manually tested for a real folder/mrl couple

This reverts commit d21a8761.

This is meant to be then plugged with removable devices

Instead of having a per-field SQL request.

Reading from the DB is "cheap", and the cache hurts the maintainability
of the code.
This removes the CachePolicy and CacheColumn field from TablePolicy.
Instead, a PrimaryKeyColumn has been added to TablePolicy

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

This is lighter, and allows us to use the interface type instead of
the implementation type as the key() method parameter.

This fixes a contraint violation exception when trying to handle
multiple tracks, with the language as their only differences.

Media now always have a single artist
Artist are being added media
The album artist meta is used to generate the list of artists
The artist meta isn't used for artists list

This makes it easier to scan a row and extract values to variables,
without having to manually pass the column index.
In the likely event we add/remove columns, this will help prevent
regressions as well.