Add remove_missing option to convert plugin

[javierbg]

Description

This PR adds a new configuration/option to the convert plugin.

The "remove_missing" option indicates that files in the convert destination directory not present in the library should be removed (asking for confirmation).

To Do

• Documentation
• Changelog
• Tests

Beware that this is my first PR here, so I'm still unaware of common practices. I tried to follow the same principles that I saw in the convert.py file, but I'm sure a lot of them will be outdated, feel free to suggest any changes.

Also, I'm still not 100% sold on the config/option name. Let me know if you have a suggestion.

[javierbg]

I see some failing checks on Windows, but it appears to be failing when invoking ffmpeg. I don't know if this is to be expected or not.

[javierbg]

Ok, that seemed to do the trick. I made the mistake of copying the setup from another test case that must have been trying to test something about the path encodings.

[codecov]

Codecov Report

❌ Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 68.83%. Comparing base (4572a7d) to head (aee3a55).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/convert.py	96.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6324      +/-   ##
==========================================
+ Coverage   68.78%   68.83%   +0.05%     
==========================================
  Files         140      140              
  Lines       18619    18644      +25     
  Branches     3054     3063       +9     
==========================================
+ Hits        12807    12834      +27     
+ Misses       5164     5161       -3     
- Partials      648      649       +1     

Files with missing lines	Coverage Δ
beetsplug/convert.py	73.68% <96.00%> (+2.70%)	⬆️

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[javierbg]

The same Windows error happens again. Can anyone help figure it out? I really don't understand why this would fail on Windows, when the "run_convert" call works in all other tests.