Fix 'from_scratch': delete all tags before writing new tags to file

[rdy2go]

Description

Fixes #3706.

Issue

Comment tags are written to file even if option 'from_scratch' is used. The same tags are not written to the file if imported together with other files as album. Therefore 'from_scratch' is not working as described in the documentation.

Solution

1. Add test: Adapt the function from the 'regular' import class and insert it in the class for the singleton import test.
2. Fix bug : Add check for 'from_scratch' option. If used, clear metadata before applying 'new' metadata with autotag.
3. No documentation change needed. Option now works as described in the documentation.
4. Add changelog.

To Do

• Documentation.
• Changelog.
• Tests.

[wisp3rwind]

Not sure what's up with CI here.

The change itself (didn't look at the test now) looks good to me: This was never implemented for SingletonImportTask, but only ImportTask, cf. #2755, and this PR only matches both implentations of apply_metadata.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.73%. Comparing base (0fec858) to head (2c240b4).
⚠️ Report is 5 commits behind head on master.

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[wisp3rwind]

Sorry for the long delay; I've just had another look at the PR in its entirety. Looks good to me!

There's one small mishap regarding the changelog, see the inline comment. Could you fix that? Then, this can be merged :)

[rdy2go]

Just resolved the merge conflict there.
Gets really tedious each time a new commit gets merged in the changelog. Is it necessary to resolve the conflicts in there with that current workflow for that file?

poe lint-docs reported "No Problems found." locally on my machine.