Improve Show handling

Since we now have a basic information extraction, we need to allow applications to list shows.

As per #165 (closed), we need to provide an "Unknown Show" to group all shows without a name. However, we need to be able to filter out this unknown show (as it may not contain anything). We also need to provide the same "is_present" handling as we do for artists & albums.

Add unknown show support
Add a nb_episode field in the Show table
Add is_present support to shows & show episode
Add shows to the sample tests
~~Add a UNIQUE constraint on show_id/episode_number/season_number~~

Edited Nov 27, 2019 by Hugo Beauzée-Luyssen

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