Improve management of instrumental files

[Neraste]

Use the instrumental_file and instrumental_track fields instead of re-discovering the instrumental file or track.

Side effect:

• The manage_instrumental method belongs now to the MediaPlayer abstract class, and is hence common to both VLC and Mpv media players;
• Dropped the dependency to filetypes.

Related to:

• Improve management of instrumental files dakara-server#190
• Improve management of instrumental files dakara-feeder#47

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.04%. Comparing base (3c72719) to head (2f280c8).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #143      +/-   ##
===========================================
- Coverage    98.04%   98.04%   -0.01%     
===========================================
  Files           16       15       -1     
  Lines         1381     1379       -2     
===========================================
- Hits          1354     1352       -2     
  Misses          27       27              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.